Robert Foster was born in 1834. After a brief schooling Foster worked in the tin industry and in his uncle's store in Indianapolis. On the outbreak of the American Civil War Foster joined the Union Army.
When Abraham Lincoln was assassinated, President Andrew Johnson ordered the formation of a nine-man military commission to try the conspirators. The commission included Foster, David Hunter, Lewis Wallace, August Kautz, Thomas Harris and Albion Howe.
On 29th June, 1865 Mary Surratt, Lewis Powell, George Atzerodt, David Herold, Samuel Mudd, Michael O'Laughlin, Edman Spangler and Samuel Arnold were found guilty of being involved in the conspiracy to murder Lincoln. Surratt, Powell, Atzerodt and Herold were hanged at Washington Penitentiary on 7th July, 1865. Surratt, who was expected to be reprieved, was the first woman in American history to be executed.
Foster left the Union Army in September, 1865. He returned to Indianapolis where he held several public offices before his death in 1903.