McKeen entered the Battle of Gettysburg still commanding 81st Pennsylvania. Col Edward E. Cross, commander of the brigade, also gave him responsibility for the 148th Pennsylvania Infantry. When Cross was mortally wounded on July 2, 1863 in the Wheatfield, Col McKeen became acting commander of his brigade. McKeen wrote the official report of the brigade's actions at Gettysburg. He records his command's role on the third day of the battle, watching the repulse of the Florida brigade under David Lang. The regiment’s memorial stands in the Wheatfield, where it did its hardest fighting. Col McKeen was back in regimental command at the Battle of Bristoe Station and the Mine Run Campaign. During an absence of Col Nelson Miles, McKeen led the brigade once more during the winter of 1863-1864.