Working PIby Isaac Peck, Publisher
The history of private investigation as a discipline is so vast and varied that even a rudimentary summary in a trade publication like Working PI risks being superficial. Still, what follows is an attempt to trace some of the building blocks and turning points that shaped the profession we know today.
While private investigators were not officially licensed in the United States until 1915—starting with California—the genesis of the profession has far deeper roots. The culture, tactics, and role of intelligence gathering in human society long predates licensing or even any formal recognition of “private investigators” as a profession. At its core, investigation reflects a timeless truth: accurate information has always carried immense value in human societies. The collection, trade, and use of information is as old as civilization itself, with it often meaning the difference between success and failure, and life and death.
Long before words like “investigator” or “detective” were conceived, societies relied on spies, informants, scouts, and information brokers. Empires, tribes, rebels, churches, organizations, and private citizens alike sought intelligence; sometimes desperately so. “Knowledge is power” has always rung true—and those who controlled information have routinely sold it, traded it, or wielded it as leverage to advance their interests. Today’s private investigators stand on a foundation that has existed for centuries: the essential human demand for information.
By the 1800s, the practice of gathering and selling information began to evolve from an informal tradecraft into a more recognizable commercial service. A century before professional licensing, this period marked the emergence of dedicated agencies and individuals who treated investigation as a vocation, and a business.
In Europe, figures such as Eugène-François Vidocq, Charles Field, and Ignatius Paul “Paddington” Pollaky established themselves as trailblazers for private investigation, initiating recognizable PI practices like systematic record-keeping, assurances of client confidentiality, and publicizing services. Meanwhile in the United States, Allan Pinkerton and the founding of the North American Detective Agency set the stage for what would become the American model of the profession.
In the century that followed, investigators in the United States and abroad expanded into nearly every arena of civic and commercial life: from domestic and infidelity cases to insurance fraud, workers’ compensation, criminal defense, business intelligence, and political power struggles. In the last few decades specifically, the profession has expanded and diversified even further, moving into digital forensics, cyber investigations, and even cryptocurrency tracing.
The modern private investigation profession rests on centuries of innovation and necessity, but its driving force has never changed: information is power, and there will always be someone willing to pay for it.
From Ancient Intelligence Agents to the Revolutionary War
Long before private detectives roamed city streets, rulers relied on human networks to collect secrets. In ancient Sumer, temple scribes and palace officials hired merchants, artisans, and servants as informers. In the wilderness of Canaan, Moses is said to have sent scouts to size up fortified cities and local resources. Such scouts were adept at mapping, getting information from locals, and preparing risk assessments; these are things private investigators still do for their clients. When Alexander the Great marched into Persia, he used scouts to gather intelligence on enemy troop movements, measure the loyalty of towns, and send coded reports back to camp.
Later, in medieval Europe, feudal lords relied on bailiffs to collect rents, deal with poachers, and put down peasant revolts (the latter a preview of one of Pinkerton’s best-known services). Crown runners crisscrossed dirt roads with royal orders but also kept an eye out for heretics and criminals. Necessity is the mother of invention when the king’s paying.
Spycraft also played a role in the revolts against kings. During the Revolutionary War, the Culper Ring operated as a covert spy network under George Washington’s direction, relaying British troop movements and plans via coded messages, dead drops, and invisible ink. Their intelligence, specifically Agent 355, helped thwart Benedict Arnold’s treason and secure American victories.
For many millennia prior to the professionalization of PIs, it’s clear some people had a knack for information retrieval, giving paying clients—or governments—the upper hand in spying and security.
The Modern PI Pioneers
Four individuals laid the groundwork for the modern private investigator. Each brought a key innovation that endures in today’s tradecraft.
Eugène François Vidocq (1775-1857) is often called the first private detective. The Parsian ex-convict turned criminal inspector pioneered systematic record-keeping, introduced plaster casts for shoeprints, and used indelible ink to track repeat offenders. Vidocq is remembered as the maker of method: he treated investigations as reproducible processes.
Allan Pinkerton (1819-1884) was the master of scaling up and then assigning down. He founded the North American Detective Agency in 1850 under the motto “We Never Sleep.” Pinkerton built a vast network of undercover agents who unearthed espionage plots during the Civil War and protected railroads during labor conflicts. His emphasis on organized surveillance teams and national manhunts set the template for large-scale, cross-jurisdictional work. His founding of the Pinkerton National Detective Agency in 1850 marked a turning point: no longer was investigative work the domain of lone operators or informal networks. Pinkerton introduced internal protocols, case documentation, and agent hierarchies—laying the groundwork for the modern investigative firm. Although he probably influenced large-scale police work more than individual PIs, his creation of large agencies provided a business template for later investigative firms.
Pinkerton’s legacy was a mindset: investigative work could be principled and procedural. Success came not from improvisation, but from replicable systems and disciplined craft. Whenever you coordinate multi-agent surveillance, log evidence with timestamps, work out client confidentiality agreements, or reach out with different services to different kinds of clients, you can thank Pinkerton. He also developed the use of mugshot archives, centralized criminal records, and informant networks, which remain foundational in both public and private sector investigating. His agency was among the first to offer scalable services across jurisdictions, from corporate security to personal protection, and even presidential detail. Pinkerton’s emphasis on methodical fieldwork and administrative rigor helped elevate the profession’s credibility and reach.
Meanwhile, two Londoners played critical roles: ex-cop Charles Frederick Field (1805-1874) opened a “civilian inquiry” office in 1852, selling his expertise to businesses and rich patrons, assuring them discretion as part of the package; while Ignatius Paul “Paddington” Pollaky (1828-1918) was a showboating self-promoter and skilled informant manager who became so famous that Gilbert and Sullivan wrote him into one of their operas, while the public devoured rumors of his exploits.
Together, Vidocq’s systems, Pinkerton’s networks, Field’s client relationships, and Pollaky’s informant-driven hustle created the four pillars of private investigation: method, scale, discretion, and human intelligence. PIs still build on this legacy.
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The Civil War and the Birth of Technological Spycraft
Between 1861 and 1865, the American Civil War became the proving ground for modern espionage. Behind the roar of musket volleys, both the Union and Confederacy raced to build spy networks, encrypt messages, steal documents, and plot kidnappings and assassinations. In doing so, they forged a template of clandestine tradecraft that private investigators would refine and repurpose for the next two centuries.
Before the U.S. government had its own intelligence service, it leaned on Allan Pinkerton’s National Detective Agency. Hired in 1861 by General George B. McClellan, Pinkerton, under the alias “Major E.J. Allen,” organized the Army of the Potomac’s first Secret Service. He intercepted Confederate dispatches, ran undercover missions deep into Southern territory, and even recruited Kate Warne, America’s first female detective.
Pinkerton stumbled at times, inflating his troop estimates and prompting McClellan to delay vital offensives. Still, Pinkerton’s centralized, civilian-staffed framework, with its surveillance teams and coded correspondence, became the blueprint for the Secret Service, the FBI, and the corporate-security industry.
Harriet Tubman, best known for the Underground Railroad, emerged as the Civil War’s most effective field operative, mapping hostile terrain, guiding the Combahee River Raid that liberated over 700 enslaved people, and using her intimate knowledge of Southern networks to supply Union commanders with actionable intelligence.
Meanwhile, detective Kate Warne foiled an assassination plot against President-elect Lincoln in Baltimore. Disguised first as a Southern socialite and later as “Mrs. Lincoln,” she infiltrated secessionist circles, uncovered a staged riot plan, and personally shepherded Lincoln through a secret night transfer, afterwards sending Pinkerton the triumphant telegram: “Plums has Nuts.”
Other agents included analyst John Babcock who turned interviews into battlefield intelligence, and the unbelievably brave operative Mary Richards Bowser, who penetrated the Confederate inner circle by going deep undercover in CSA President Jefferson Davis’s home, disguised as a servant.
Both sides in the Civil War also pioneered tools still in use today: encrypted telegraph ciphers, invisible ink, aerial reconnaissance, photographic reproduction, and purpose-built cipher disks. These innovations, born of battlefield necessity, cemented a legacy of espionage that would scale down into the private investigation profession.
Criminal Defense
While popular history often celebrates investigators as crime-fighters, an equally important legacy comes from those working for criminal defense attorneys—ensuring that those charged with a crime in this country are given a fair shot at justice.
By the mid-20th century, defense attorneys began increasingly relying on private investigators to challenge expert testimony, reconstruct crime scenes, and identify inconsistencies in police reports. These professionals evolved from mere fact-gatherers into analysts, technologists, and strategic thinkers. Surveillance, undercover operations, and forensic analysis became standard tools in countering prosecution narratives.
Over time, trade journals and legal literature began to recognize the defense investigator as “the eyes and ears of the defense attorney,” responsible for gathering all pertinent information—favorable and adverse. Criminal defense work also shaped the business dynamics of the PI profession, introducing retainer models, specialization, and agency structures tailored to legal clients.
One of the most transformative figures in this evolution was Harold “Hal” Lipset (1919-1997). Often compared to Steve Jobs, Lipset revolutionized investigative technology and challenged conventional norms. Active in San Francisco, he worked with high-profile clients like Angela Davis and Jim Jones and even inspired the character Harry Caul in Francis Ford Coppola’s The Conversation. Lipset’s innovations in miniature surveillance— such as the iconic bug hidden in a martini olive—redefined covert operations and raised ethical questions about privacy.
Lipset recruited intellectual operatives, including former philosophy professor Josiah Thompson (born in 1935), who later became a respected defense investigator. Thompson’s work helped free wrongfully convicted Chol Soo Lee and exposed systemic flaws in law enforcement’s approach. Together, Lipset and Thompson helped redefine private investigation as a sophisticated, strategic, and ethically engaged profession. While Lipset is a famous investigator for myriad reasons, he once opined that there was “no greater thrill in the world” than defending accused criminals and proving them innocent.
There’s another reason why private investigators have leaned into criminal defense. The Sixth Amendment to the U.S. Constitution guarantees the right to legal counsel for criminal defendants, and for over 50 years, the prevailing opinion has been that fulfilling this right includes giving criminal defendants access to private investigative work. This was made clear by several legal opinions and scholarly articles that came up after Gideon v. Wainwright (1963), the case where the Supreme Court ruled that states must provide legal counsel to indigent defendants in felony cases.
For example, in a 1970 Cornell Law Review (Volume 55, Issue 4) article, law professor Craig Bowman warned that denying defendants investigative resources risks undermining the very idea of equality before the law. The government, he argued, should not “allow poverty to create an imbalance in the administration of criminal justice.” Instead, Bowman concluded, “to ensure balance, the government should provide the indigent defendant with the services of investigators and experts to develop, prepare, and present his defense.”
Beginning in the mid-to-late 20th century, the business of criminal defense investigation received a boost from a series of state court decisions that greatly expanded the government’s mandate to aid poor and indigent criminal defendants. These included Louisiana’s finding in State v. Madison (1977) that the right to an investigator may “in many cases be an adjunct to the right to counsel” and that “when an indigent defendant shows that his attorney is unable to obtain existing evidence crucial to the defense, the means to obtain it should be provided for him, and if the indigent defender system cannot defray the expense, the State ought to supply the funds.” California issued strong judicial opinions concerning “a criminal defendant’s right to ancillary defense services,” and providing for affidavits specifying the necessity of “funds for the specific payment of investigators, experts, and others for the preparation or presentation of the defense.”
Other states reached similar conclusions, resulting in a rare consensus on defendants’ rights that hasn’t required the U.S. Supreme Court to pronounce. Since prosecutors enjoy far easier access to things like forensic laboratory work, a defendant’s access to investigative expertise is vital, making it necessary for defense attorneys to hire PIs with experience in criminal investigation.
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Fault-Based Divorce: Beginning of Domestics
As fault-based laws dominated at the turn of the 20th century, PIs became courtroom linchpins, determining whether a marriage could legally dissolve or not—becoming a lucrative business. The evolution of private investigative work owes much to this era, which required petitioners to prove misconduct—typically adultery, abandonment, or cruelty—to obtain a legal separation. In a legal landscape where personal dissatisfaction was insufficient grounds for divorce, concrete evidence was paramount. This evidentiary burden created a niche for PIs, whose ability to uncover actionable proof became indispensable.
PIs refined specialized techniques to meet these demands for their clients that are still used to this day. Surveillance, covert photography, document retrieval, and witness interviews became standard tools of the trade. Investigators tracked suspect spouses, documented extramarital affairs, and traced hidden assets, ensuring that their findings could withstand judicial scrutiny. Divorce, once considered an “exceptional remedy” governed by religious and cultural taboos, required rigorous proof—something only skilled professionals could reliably provide. Divorce cases opened a lucrative and emotionally charged frontier, offering a steady stream of clients desperate for evidence.
In 1969, California became the first state to enact a no-fault divorce law, allowing couples to cite irreconcilable differences rather than proving fault. By the early 1980s, most states had adopted similar statutes, fundamentally transforming American family law.
The rise of no-fault divorce laws transformed the investigative landscape. Courts no longer required proof of wrongdoing to grant a divorce, shifting the focus to financial, custodial, and reputational concerns. Investigators adapted by mastering digital forensics, asset tracing, and child custody documentation. While discovering adultery remains central to what PIs today refer to as “domestics,” PI work in ongoing divorce work is even more varied and includes asset locates, surveillance, character backgrounds, and more.
Insurance Related Work
An entire article (or even a book) could be written about the history of insurance and financial fraud, and unsurprisingly it goes way back: in 300 B.C. a Greek sea merchant named Hegestratos accidentally drowned himself post-escape after his crew caught him trying to execute a scheme to sink his ship and sell the cargo he’d borrowed money against.
But the twentieth century brought a vast leap in financial activity, and capitalism spread on a mass scale. The insurance industry grew extensively after the Second World War. The postwar economic expansion created a need for consumer insurance policies covering everything from automobiles to life and property. The growing popularity of life insurance during the early 20th century meant that PIs were frequently called upon to do background investigations—and the foundation of the major credit reporting agencies was set.
PIs were also regularly employed to investigate staged accidents, falsified damage reports, and inflated claims. Such cases are often complicated, but as the early wave of fraud detection focused largely on property and casualty insurance, investigators would photograph damaged vehicles, interview witnesses, and track claimants whose stories didn’t quite add up. As the insurance landscape evolved, so did the nature of fraud, including in Workers’ Compensation, which emerged as a concern beginning in the 1970s. Companies were paying billions of dollars annually for disability and medical benefits, and needed help verifying the legitimacy of workplace injuries. PIs surveilled claimants to see if they were truly incapacitated, or if outside of work they were magically running, lifting, etc. Long-term surveillance, public records research, and accident reconstruction all emerged from this period.
Today, such investigations cover everything from disability claims to employment verification. In fact, insurance-related investigations remain one of the most stable and specialized sources of PI work, a multibillion-dollar phenomenon requiring specialists to uncover deception and protect insurers from financial loss. What began as a reactive measure has become a cornerstone of investigative practice.
Corporate Clients
Beginning in the 1950s, private investigators increasingly found themselves enlisted not just by individuals or attorneys, but by corporations seeking to protect their assets, outmaneuver competitors, and root out internal threats. As the postwar economy surged and industries expanded, so did the stakes of proprietary information, from trade secrets to client lists, manufacturing processes to strategic plans. In this climate, PIs became the investigative arm of corporate America.
Private investigators weren’t just chasing cheating spouses and insurance fraudsters but were now uncovering employee embezzlement, sniffing out information on competitors, and doing deep background work on executives. Rival companies sometimes planted insiders, and investigators were hired to find them. This sometimes involved pushing the limits of what was legally allowed. Legal or not, these tools gave companies new ways to track internal communications and stop information leaks.
Since then, the rise of digital infrastructure and globalization have made corporate espionage even more complex. Developments in cyber-forensics, data recovery, and network infiltration have followed, as methods and techniques have hurried to keep up with new scenarios.
New Technology, Old Values
Private investigators have always been innovators in using the technology of their day. As technologies have accelerated in the last century, so too have investigative tools expanded at an exponential pace. The transition from analog and paper to digital and electronic, the introduction of GPS, and countless other advances, while often intended for other fields, fundamentally shifted the possibilities of investigative work.
By the 1970s and ’80s, PIs were already employing portable audio recorders, sophisticated telephoto lenses, tiny bugs, and vehicle tracking systems. Then came a breakthrough from the U.S. Department of Defense: the 1978 launch of the Block-I GPS satellite. Initially restricted to military use, GPS became available to investigators after President Bill Clinton’s 1996 directive opening the technology up to civilian use. This was a watershed moment for the profession.
Consumer electronics also played their part. Camcorders, pagers, and early mobile phones enabled real-time coordination, instant research, and more accurate documentation. As these technologies developed, PIs learned how to use them, and sometimes how to hack them. In the 2010s, PIs in New York and San Francisco made headlines getting busted for hacking Skype, stealing email passwords, and breaking into databases while seeking classified information on police officers.
What distinguishes the modern era, however, is not just new tools but the sheer breadth of arenas where investigators now work. From insurance claims and divorce cases to cryptocurrency tracing, cyber intrusions, personal brand protection, corporate due diligence, and international asset recovery, today’s profession spans a digital landscape that is global, fast-moving, and infinitely diverse. If earlier generations of investigators were bound by geography and manpower, today’s PIs operate across networks, currencies, and continents—meeting new challenges with the same ingenuity their predecessors brought to locks, ledgers, and footprints in the mud.
Conclusion
Private investigation has never been about tools alone. From ancient scribes and scouts to Vidocq, Pinkerton, and today’s digital forensic experts, the profession has always adapted to new technologies and new demands. What has not changed is the enduring truth that information is power—and society will always need those willing to uncover it.
Today’s investigators inherit not only technological advancements, but also a lineage of grit: ingenuity, discretion, and resilience that stretches back thousands of years. Heirs to the spies, scouts, and informants of generals and kings, as well as pioneers who professionalized the craft, today’s PIs now deal with the seemingly unlimited scope of information and technology: evidence may be hidden in a blockchain wallet, a cloud database, a corporate server on another continent, or a social media post.
As we look to the future, one thing is certain: the need for investigation endures, and so will those who answer it.
About the Author
Isaac Peck is the Publisher of Working PI magazine and the President and Senior Broker of OREP.org, a leading provider of liability insurance for PI professionals. Working PI is the most widely read print magazine for investigators nationwide, reaching over 25,000 PIs. Investigators who become OREP Members enjoy two CE courses (15 hours of education) at no charge (Visit for details). Isaac brings over 10 years of experience leading teams in Professional Liability insurance underwriting, operations, technology, and marketing—with a focus on E&O and general liability insurance for professionals. He holds a Master’s Degree in Accounting. Reach Isaac by phone at (888) 347-5273 or email at isaac@orep.org. CA License #4116465.
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