In 1833 Eugène François Vidocq, a French soldier, criminal and privateer, founded the first known private detective agency, "Le Bureau des Renseignements Universels pour le commerce et l'Industrie"  (Office of Intelligence) and hired ex-convicts. Official law enforcement tried many times to shut it down. In 1842 police arrested him in suspicion of unlawful imprisonment and taking money on false pretences after he had solved an embezzling case. Vidocq later suspected that it had been a set-up. He was sentenced for five years with a 3,000-franc fine but the Court of Appeals released him. Vidocq is credited with having introduced record-keeping, criminology and ballistics to criminal investigation. He made the first plaster casts of shoe impressions. He created indelible ink and unalterable bond paper with his printing company. His form of anthropometrics is still partially used by French police. He is also credited for philanthropic pursuits – he claimed he never informed on anyone who had stolen for real need.

After Vidocq, the industry was born. Much of what private investigators did in the early days was to act as the police in matters that their clients felt the police were not equipped for or willing to do. A larger role for this new private investigative industry to was to assist companies in labor disputes. Some early private investigators provided armed guards to act as a private militia.

In the U.S., the Pinkerton National Detective Agency was a private detective agency established in 1850 by Allan Pinkerton. Pinkerton had become famous when he foiled a plot to assassinate then President-Elect Abraham Lincoln. Pinkerton's agents performed services which ranged from undercover investigations and detection of crimes to plant protection and armed security. It is sometimes claimed, probably with exaggeration, that at the height of its existence the Pinkerton National Detective Agency employed more agents than the United States Army.

During the labor unrest of the late 19th century, companies sometimes hired operatives and armed guards from the Pinkertons and similar agencies to keep strikers and suspected unionists out of their factories. The most famous example of this was the Homestead Strike of 1892, when industrialist Henry Clay Frick hired a large contingent of Pinkerton men to regain possession of Andrew Carnegie's steel mill during a lock-out at Homestead, Pennsylvania. Gunfire erupted between the strikers and the Pinkertons, resulting in multiple casualties and deaths on both sides. Several days later a radical anarchist, Alexander Berkman, attempted to assassinate Frick. In the aftermath of the Homestead Riot, several states passed so-called "anti-Pinkerton" laws restricting the importation of private security guards during labor strikes. The federal Anti-Pinkerton Act of 1893 continues to prohibit an "individual employed by the Pinkerton Detective Agency, or similar organization" from being employed by "the Government of the United States or the government of the District of Columbia."

Pinkerton agents were also hired to track western outlaws Jesse James, the Reno brothers, and the Wild Bunch, including Butch Cassidy and the Sundance Kid. The Pinkerton agency's logo, an eye embellished with the words "We Never Sleep," inspired the term "private eye."

It was not until the prosperity of the 1920s that the private investigator became a person accessible to the average American. With the wealth of the 1920s and the expanding of the middle class came the need of middle America for private investigators.

Since then the private detective industry has grown with the changing needs of the public. Social issues like infidelity and unionization have impacted the industry and created new types of work, as has the need for insurance and, with it, insurance fraud, criminal defense investigations and the invention of low-cost listening devices. In a number of countries, a licensing process has been introduced that has put criteria in place that investigators have to meet: in most cases, a clean criminal record. This has combined with modern business practices that have ensured that most investigators are now professional in outlook, rather than seeing the PI world as a second career opportunity for retired policemen.