Here's a story I've been telling a brief version of on tours lately:
In 1910, a robber named Thomas Jennings was making a run from the scene of a burglary - the Englewood home of Clarence Hiller, who had once been chief clerk of a local railway. During his escape, Jennings fired two shots at Hiller and killed him instantly. But Jennings made a mistake he couldn't have predicted at the time: he touched some wet paint on a railing with his hand, leaving a print. After his capture, the prosecution used the fingerprint to convict him. Such a thing had never been done before.
Naturally, the defense objected to such evidence. The attorney told judge Kavanagh that "There is only one case in the world where finger prints were attempted to be introduced in a criminal trial. That was an old English case, and it was then held that a special law would have to be passed to legalize such evidence."
Captain Michael Evans of the police was asked about his confidence in the system. "Suppose," said the attorney, "that I were to plant my thumb - so - on this piece of white paper, could you make a print from this paper that would prove it was my thumb that had been pressed on it?"
Upon replying that he could, the attorney said (probably with a smug grin), "All right. Go ahead and do it." Evans dusted the spot the attorney had touched with powder and showed the jury the print, and a print taken from the attorney matched the one on the paper.
But the judge and jury were both convinced, and Jennings was sentenced to hang.
I have no doubt that earlier cases of finger prints being used as evidence could probably be brought up by a thorough search (this strikes me as one of those fields where the answer to "which was the first" or "what was the real cause" depends greatly on who you ask; the police seem to have used it for some time), but the idea of using them as actual evidence certainly seemed novel at the time. The Tribune pondered what this would mean for the future:
WILL CRIMINALS WEAR GLOVES?
Now that the criminal classes are menaced with the finger print system, George Porteous, an expert in criminology, propounds this question: "Will the future housebreaker wear gloves to foil the police in procuring finger print evidence?....One pair of gloves of a certain make is like another. But it is different with a person's hands or fingers. No two hands are exactly alike. If Jennings had worn gloves the night he entered the Hillers' house I don't think he would have been convicted."
In any case, the defense appealed, and Jennings got a couple of last minute reprieves from hangings before the Supreme Court of Illinois decided that finger prints were admissible in December, 1911. In the busiest day in the history of the Chicago gallows, he was hanged along with four other men in February, 1912. He had to be carried by the armpits to the scaffold. The bodies of the other four men (who, unlike him, were white) were brought to an undertaking parlor on Noble and Division) where a large crowd had come to see what was happening. "Three thousand persons," the Tribune wrote, "viewed the remains and then lingered in the street so as to miss none of the misery of the dead men's relatives."
A whole lot more about hangings is in William Griffith's FATAL DROP: TRUE TALES OF THE CHICAGO GALLOWS.