During his final year in Congress, the senior Russell secured an appointment to the United States Military Academy for his son.

In 1862, Russell was promoted to major in the regular army and assigned to the U.S. 8th Infantry Regiment.

Later in 1862, Russell was promoted to brigadier general of volunteers and commanded a brigade during the Rappahannock campaign.

Russell was primarily in reserve during the Battle of Gettysburg, but was brevetted colonel in the regular army shortly afterward.

On May 3, 1867, President Andrew Johnson nominated Russell for the grade of brevet major general in the regular army, to rank from the date of his death in the field, September 19, 1864, and the United States Senate confirmed the appointment on February 14, 1868.