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In January, Captain Sinex succeeded Lieutenant Colonel Wallace, and Captain Lentz succeeded Major Todd. The regiment remained in camp, picketing the rear line. They built corduroy roads and moved artillery and trains on Burnside's Mud March. When the Mud March was abandoned, they returned to camp. They remained in camp through February and March.
In April, the 91st guarded United States and Banks' Fords for several weeks. At the end of the month, the Chancellorsville campaign started. After protecting the army's rear as it crossed Kelly's Ford, the Fifth Corps (including the 91st) marched toward the Chancellor House. In May, the Chancellorsville campaign continued. After building defensive works near the Chancellor House, the 91st and its brigade served as support for General French. They attacked the Confederates through a woods, but were outflanked and ran out of ammunition. At the end of the campaign, the Fifth Corps served as rear guard for the army. They then returned to their old camp. The regiment was transferred to the 3rd Brigade (Col. O'Rourke), 2nd Division (General Sykes).
In June, the regiment (and the whole Army of the Potomac) followed Lee as he went north, through Manassas Junction, Gum Spring, Aldie, Edwards' Ferry, and Frederick City. They rested there until the 28th, when they began moving north again. In July, they arrived at Hanover, and then marched through the night to Gettysburg. They took part in the Battle of Gettysburg. They then moved parallel to the Confederates through Marsh Creek, Utica, Middletown, Boonsboro, and Antietam Creek, but the Confederates escaped.
In August and September, they had duty on the Rappahannock. The Bristoe Campaign took place in October. In November, they advanced to the line of the Rappahannock, and took part in the Mine Run Campaign.
In December, most of the regiment re-enlisted; the remainder were temporarily transferred to the 155th Pennsylvania.