Samuel Arnold was born in Washington D. C. on September 6, 1834. The Arnold family later moved to Baltimore. Arnold attended St. Timothy's Military Academy, where he was a classmate of John Wilkes Booth.
In the late summer of 1864, Booth recruited Arnold, then unemployed and bored, to join the conspiracy to kidnap Lincoln and take him to Richmond. On March 15, 1865, Arnold met Booth at Gautier's Restaurant in Washington to plan the kidnapping, scheduled for two days later. When Lincoln cancelled plans to attend a play at the Campbell Hospital on March 17, the abduction plans fell through and Arnold returned to Baltimore.
A March 27 letter from Arnold to Booth was discovered by investigators during a search of Booth's hotel room after the assassination. On April 17, authorities arrested Arnold in Old Point Comfort, Virginia, where he worked as a clerk.
In the 1865 Conspiracy Trial, prosecutors introduced Arnold's March 27, 1865 letter to Booth. In his letter, Arnold wrote that "None, no, not one were more in favor of the enterprise than myself." Arnold's attorney, Walter Cox, argued that Arnold "backed out from this insane scheme of capture" and it was "abandoned somewhere about the middle of March." Arnold, he argued, left Washington for Maryland about March 20 and that there "is no evidence that connects" Arnold with the "dreadful conspiracy" of assassination. Cox told the Commission that Arnold's participation in the "mere unacted, still scheme" of abduction was "wholly different from the offense described in the charge."
The Military Commission found Arnold guilty and sentenced him to life in prison. President Andrew Johnson pardoned Arnold on March 1, 1869.
After his release from prison, Arnold wrote a detailed confession of his role in the plot to kidnap Lincoln. His statement was published in Samuel Bland Arnold: Memoirs of a Lincoln Conspirator by Michael W. Kauffman. Arnold died on September 21, 1906 of tuberculosis.