After IX Corps was transferred west, Curtin led the 45th Pennsylvania in the Siege of Vicksburg.

When IX Corps returned to the Army of the Potomac in 1864, Curtin led his regiment at the Battle of the Wilderness in the 2nd Division of Brig.

Curtin took command of 1st Brigade, 2nd Division, on May 11, 1864, after Col. Zenas Bliss was injured at the Battle of Spotsylvania Court House.

After the Confederate surrender, in the Department of Washington, Curtin led the 3rd Division, IX Corps, in place of Brig.

On December 20, 1864, President Abraham Lincoln nominated Curtin for appointment to the grade of brevet brigadier general of volunteers, to rank from October 12, 1864, and the United States Senate confirmed the appointment on February 14, 1865.