Pinkerton's career as a detective began by chance when he discovered a gang of counterfeiters operating in an area where he was gathering wood. His assistance—first in arresting these men and then another counterfeiter, led to his appointment as deputy sheriff of Kane County, Illinois, and, later, as Chicago's first full-time detective.
Pinkerton left his job with the Chicago police force to start his own detective agency. One of the first of its kind, this predecessor to Pinkerton's National Detective Agency, provided an array of private detective services—specializing in the capture of train robbers and counterfeiters and in providing private security services for a variety of industries.
The first Pinkerton office opens at 80 Washington Street, Chicago, IL in 1850.
By the 1870s, Pinkerton's growing agency had accumulated an extensive collection of criminal dossiers and mug shots that became a model for other police forces.
In 1861, while investigating a railway case, Pinkerton uncovered an apparent assassination plot against Abraham Lincoln. It was believed that conspirators intended to kill Lincoln in Baltimore during a stop along the way to his inauguration. Pinkerton warned Lincoln of the threat, and the president-elect's itinerary was changed so that he passed through the city secretly at night.
After McClellan was replaced as the commander of the Army of the Potomac in 1862, Pinkerton resumed the management of his detective agency. The agency expanded after the Civil War, opening offices in New York City (1865) and Philadelphia (1866). As his business grew, Pinkerton drew public attention to its work by producing a series of popular "true crime" stories.
In time, because Pinkerton's Agency was often hired by industrialists to provide intelligence information on union-organizing efforts, Pinkerton guards and agents gained notoriety as strikebreakers. Notable confrontations between Pinkerton agents and laborers include the 1886 Haymarket Riot and the 1892 Homestead Strike, both of which occurred after Pinkerton's death in 1884.