Walker remained at the Berkeley Plantation until his promotion on August 11 to major and transferral with General Couch to the II Corps of the Army of the Potomac. Although the II Corps later saw action at the battles of Antietam and Fredericksburg, the latter being under the new command of Maj. Gen. Ambrose Burnside, Walker and the Corps did not join Burnsides's Mud March over the winter. Walker was promoted to lieutenant colonel on January 1, 1863, and remained with the II Corps. He fought the Battle of Chancellorsville in May 1863, where his hand and wrist were shattered and neck lacerated by an exploding shell. A record of the 1880 Census indicated that he had "compound fracture of the metacarpal bones of the left hand resulting in permanent extension of his hand". Later in 1896, as the President of MIT, he would receive one of the first radiographs in the country, which documented the extent of the damage to his hand. He did not return to service until August 1863. Walker participated in the Bristoe Campaign and narrowly escaped encirclement during the Battle of Bristoe Station before withdrawing and encamping near the Berry Hill Plantation for much of the winter and spending some leave in the North.