On April 4, 1864 he was promoted to brigadier general of U.S. Volunteers, to rank from November 29, 1862. He was placed in command of the 3rd Brigade, 3rd Division in William T. Sherman’s XV Corps. Matthies led this brigade during the Vicksburg Campaign, fighting at the Battle of Jackson and in the assaults on Vicksburg. During the following siege, Matthies’ brigade was transferred to the 7th Division of the XVII Corps.