Kate Warne, America's First Woman Detective

In 1856, 23-year-old widow Kate Warne answered an employment ad for the Pinkerton Detective Agency. She had to convince Allan Pinkerton that she could gain information in ways that a man couldn't, and she was right. Warne's success at gathering intelligence led Pinkerton to hire more woman detectives, and Warne was put in charge of them. During the Civil War, Pinkerton agents were charged with infiltrating Confederate society to gather information on troop movements

It was in this second role that Warne helped to prevent an assassination attempt on President Abraham Lincoln. By this time, Warne was the superintendent of all of Pinkerton’s woman detectives, but he called on her especially to pose as a Southern lady in Baltimore and help learn details of the suspected plot.
“Mrs. Warne was eminently fitted for this task. Of rather a commanding person, with clear-cut, expressive features, and with an ease of manner that was quite captivating at times, she was calculated to make a favorable impression at once,” Pinkerton wrote in his book The Spy of the Rebellion. “She was a brilliant conversationalist when so disposed, and could be quite vivacious, but she also understood that rarer quality in womankind, the art of being silent.”
Warne won over the wives of several conspirators, gaining key information to uncover their scheme to kill Lincoln while he traveled by train and destroy a section of track as well. She then aided Pinkerton himself in smuggling the president secretly aboard a train so that he could pass through Baltimore undetected.  

Warne's story is only the first of 10 Trailblazing U.S. Law Women you can read about at mental_floss.