Pinkerton fell into detective work by accident. A Scottish immigrant to America, he first worked as a cooper for a brewery outside Chicago. While in the woods looking for appropriate material for barrel staves, he stumbled upon a gang of counterfeiters. Pinkerton notified the local sheriff and helped him make the arrests. Pinkerton eventually took on more law enforcement work and was known for his strong abolitionist views.
Moving to Chicago, he gained additional fame when he helped helped rescue two abducted Michigan girls. Pinkerton tracked the outlaws and shot one of the kidnappers. This incident in 1853, provided him a job as the Cook County Deputy Sheriff. Realizing the potential for work, Pinkerton formed his own private agency, the Pinkerton National Detective Agency and created as it's motto the phrase, "We never sleep."
Because of his strong abolitionist views and his fame in Chicago, he was hired to guard Abraham Lincoln, where he foiled an assassination plot on the way to the inauguration. Pinkerton gained a reputation for professionalism and his dogged pursuit of criminals. This reputation was encouraged by dime-store novels and Pinkerton's own writings.
(Pinkerton, on left, with Lincoln)
Pinkerton pioneered a number of then unconventional detective techniques including tailing a suspect and going undercover. Even the term Private Eye comes from the Pinkerton logo.