Posey was promoted to brigadier general on January 18, 1863, to rank from November 2, 1862. The following May, Posey's brigade saw limited action at the Battle of Chancellorsville, maintaining a reserve position at Salem Church. During the army reorganization following Stonewall Jackson's death, Posey's brigade was assigned to Major General Richard H. Anderson's division of the Third (A.P. Hill's) Corps. During the Battle of Gettysburg in July, the brigade was part of Anderson's July 2 attack on Cemetery Ridge, conducting a "feeble, disjointed attack that was repulsed."