[1][2] He is not to be confused with another Union officer, Brevet Brigadier General Charles H. Tompkins (d. 1895) who commanded the 1st Rhode Island Light Artillery Regiment.

It was in this unit that he gained fame for his heroism in action at the Battle of Fairfax Court House (June 1861) while a 1st lieutenant, to which he was promoted on April 30, 1861.

Tompkins was appointed as one of the nine officers assigned to the military commission investigating the conspirators involved in President Lincoln's assassination.

The position of Assistant Quartermaster General, carried a rank of colonel in the Regular Army, to which Tompkins was promoted the same day.

His eldest son, Selah Reeve Hobbie (“Tommy”) Tompkins, was an Army officer who became Colonel of the 7th Cavalry Regiment.

His second son, Colonel Frank Tompkins, also served as a career army officer including notable participation as a major in the 13th Cavalry in both the raid on Columbus, New Mexico by Pancho Villa and the Punitive Expedition that followed for which he was awarded the Distinguished Service Cross.