Born in Georgetown, D.C., Key was the son of Francis Scott Key and the great-nephew of Philip Barton Key. He was also a nephew of Chief Justice Roger B. Taney. He married Ellen Swan, the daughter of a Baltimore attorney, on November 18, 1845. Allegedly the handsomest man in Washington and by 1859 a widower with four children, Key was known to be flirtatious with many women. engraving of Daniel Sickles shooting Key Key was appointed to his father's former position, United States Attorney for the District of Columbia, by President Pierce in September 1853, during a recess of the Senate; the Senate later confirmed his nomination in March 1854. Four years later, he was nominated, and confirmed again, for another four-year term; thus, he would serve until his death. engraving of Mrs. Sickles from a photograph of Mathew Brady Some time in the spring of 1858, Teresa Sickles began an affair with Key. Dan Sickles, though a serial adulterer himself, had accused his much-younger wife of adultery several times during their five-year marriage, but she had repeatedly denied it to his satisfaction. But then Sickles received a poison pen letter informing him of his wife's affair with Key. He confronted his wife, who confessed to the affair. Sickles then made his wife write out her confession on paper. Sickles saw Key sitting on a bench outside the Sickles home on February 27, 1859, signalling to Teresa, and confronted him. Sickles rushed outside into Lafayette Square, cried "Key, you scoundrel, you have dishonored my home; you must die", and with a pistol repeatedly shot the unarmed Key.
Death
Key was taken into the nearby Benjamin Ogle Tayloe House, where he died some time later. Sickles was acquitted on the basis of temporary insanity, a crime of passion, in one of the most controversial trials of the 19th century. It was the first successful use of the defense in the United States. One of Sickles' attorneys, Edwin Stanton, later became the Secretary of War. Newspapers declared Sickles a hero for "saving" women from Key. Years later, while attending the theater in New York City, Sickles became aware of the presence of Key's son, James Key, in the audience; both men watched each other throughout the performance. Nothing happened. Key is buried in Oak Hill Cemetery, with a dedicatory in his son-in-law's family plot in Westminster Hall and Burying Ground in Baltimore.
