Prior to the Chancellorsville campaign, he was relieved from duty with the Army of the Potomac and assigned to the defenses of Washington where he returned to his seat in Congress.

During the Union assaults on Petersburg, Gilman assumed command of the 1st Division in the XVIII Corps on the last day of battle.

Thereafter he commanded the Union troops on the north side of the James River with his headquarters located at Fort Pocahontas.

Marston was elected to the Thirty-ninth Congress (March 4, 1865–March 3, 1867) and in 1870 declined the Governorship of Idaho Territory.

On March 4, 1889, Marston was appointed to the U.S. Senate to fill the vacancy in the term commencing on that date and served until June 18, 1889, when a successor was elected.