After the battle, the division became 3rd Division of the VI Corps under Brig. Gen. John Newton. Eustis participated in the Battle of Fredericksburg in the brigade of Brig. Gen. Charles Devens. He then served under Col. William H. Browne in the Chancellorsville campaign. When Browne was wounded at the Battle of Salem Church, Eustis succeeded to command of the brigade. He led the brigade at the Battle of Gettysburg, where his units were in reserve. Eustis became a brigadier general on September 12, 1863. He served in the autumn campaigns of 1863 under Brig. Gen. Henry Dwight Terry, who had taken over after Newton became commander of I corps.