Cox was the attorney for the Lincoln Assassinators Samuel Arnold and Michael O’laughlin. (The Military Commission found Arnold guilty and sentenced him to life in prison. President Andrew Johnson pardoned Arnold on March 1, 1869. The Military Commission found O'Laughlen guilty and sentenced him to life in prison. He died two years later in prison at Fort Jefferson.) On April 16, 1868, Cox testified in the impeachment trial of President Andrew Johnson, having been called as a witness by Johnson's defense team. During Cox's service, he presided over the trial of Charles J. Guiteau, who was convicted of the assassination of President James A. Garfield.