The family later moved to Baltimore where Arnold attended St. Timothy's Hall, a military academy – where he and John Wilkes Booth were schoolmates.

[4] After his discharge, Arnold returned to Baltimore and in the late summer of 1864, he was recruited by Booth to be part of the kidnap plot.

[4] Arnold and the other alleged conspirators, John Wilkes Booth, David Herold, Lewis Powell, Michael O'Laughlen, and John Surratt, were to kidnap Lincoln, ferry him through Van Ness Mansion owned by Thomas Greene, and hold him to exchange for the Confederate prisoners in Washington D.C.

He admitted his part in the plot to kidnap Lincoln, and his co-workers supported his contention of being in Virginia at the time of the assassination.

Arnold was found guilty of conspiracy by a military tribunal and sentenced to life in prison at Fort Jefferson, along with Samuel Mudd, Michael O'Laughlen, and Edmund Spangler.