Of course Sherlock Holmes would be the most famous one, but unfortunately he’s a fictional character created by Sir Arthur Conan Doyle.
One of the first most famous Private Detectives was Allan Pinkerton (1819 – 1884). He was by the side of Abraham Lincoln during the civil war. One of his more famous achievements was foiling an assassination plot against President Lincoln in Baltimore, Maryland. What many people don’t know is that he the father of several major investigative techniques such as surveillance of a suspect, known as ‘shadowing’ at the time, and also undercover work (he called it “assuming a role”).
Pinkerton served as the head of the Intelligence Service from 1861-1862 and used his men for undercover operations. They would pretend to be sympathizers of the Confederacy in order to gain vital military intelligence.
He also created the Pinkerton National Detective Agency. Some say that at one point it employed more men than the United States Army. The darker side of Pinkerton’s history included suppression of labor riots. Even to this day certain labor organizers use the word “Pinkerton” to describe those who side with the management.