Mann spent the next winter in Chicago Union Army recruiting office; recruits were sent to fill the ranks of the 39th. In 1864 the regiment was reassigned to the Army of the James under Colonel Thomas O. Osborn. Mann assumed Osborn's duties after he was injured at the Battle of Drewry's Bluff. The 39th made an advance against a Confederate defensive position and captured a brigadier general. For his actions, Mann himself was brevetted brigadier general.