Born on August 25th, 1819 in Glasgow Scotland, Pinkerton was the son of a Scottish police sergeant. He grew up in a lower class section of Glasgow. Pinkerton moved to the United States in the early 1840's. He settled in Dundee Illinoise. He was a leader in the abolitionist movement and worked for a short time with an underground railroad that brought in slaves from the south. Pinkerton was very outspoken as an abolitionist but a great deal of his activities were covert in nature. At about this time, counterfeiting flourished in this area at an alarming rate. It was so bad it was effecting both the local and national economy. Pinkerton accidently discovered a counterfeit camp headquarters on an island in the middle of a lake that later became known as Bogus Island. Pinkerton investigated the matter and ended up successfully rounding up the gang. It was the largest counterfeit ring ever caught in the United States. He obtained a great deal of local notoriety and was appointed Deputy Sheriff. He also became Chicago's first police detective in 1850. That same year, he started Pinkerton National Detective Agency. Although there were a few private investigative agencies around at the time, most had bad reputations. Pinkerton was the first to maintain a good reputation. He was the only agency at the time that set uniform fees and did not take on divorce cases.
Pinkerton quickly established himself in the investigative business. His main work consisted of running down railroad thiefs. His first investigative business was a partnership with a prominent Chicago attorney E. Rucker. The partnership was dissolved within a year and Pinkerton operated the agency himself. The Pinkerton Agency became well known for it's investigative work in the area of union activities. Pinkerton is given credit for keeping unions out of various trades in the Chicago area for years.
While performing a security measure for one of his railroad company clients, Pinkerton ran across a plot to have the newly elected President Lincoln assassinated before he reached Washington. The President elect was already on his way from Springfield to Washington D.C. to take his oath of office. The plot was uncovered by Pinkerton while Lincoln was in Philadelphia. Pinkerton requested a meeting between himself and Lincoln which was granted. In the meeting Pinkerton said,

" We have come to know, Mr. Lincoln, and beyond the shadow of a doubt, that
there exists a plot to assassinate you. The attempt will be made on your way
through Baltimore, day after tomorrow." As Pinkerton went on, Lincoln
listened and interrupted with many questions.

After explaining the plot to Lincoln, Pinkerton told Lincoln that he should go with him directly to Washington D.C. "this very night". Lincoln explained that he was supposed to give a speech at the state assembly tomorrow and another speech at Independence Hall and could not go on this date. Lincoln did agree, however, to follow any of Pinkerton's instructions after the next day's events.
In 1859 Pinkerton was brought in to recover a missing package that contained $40,000 in it. He traveled to New York to meet his client. It was the practice of the day for investigators to obtain a percentage of recovery on such matters. Pinkerton always felt that such fee structures were not right and had long established a policy of having a set hourly, daily, and weekly fee which was explained to his clients in advance. Other investigators had tried to handle this case but all were unsuccessful. Pinkerton worked hard on this case as he wanted to develop a reputation in the New York area. This was his first big case in the East. Within one year, Mr. Pinkerton had recovered the package.

During the Civil War, the United States Government contracted with Pinkerton to spy on the South. Actually, Presdeint Lincon wanted to set up a Secret Service, a federal investigative agency, but felt that from a political standpoint, a federal police force would not be a good political move.