At the Battle of Gettysburg, the III Corps commander was the notorious Maj. Gen. Daniel Sickles, under whom Birney command the 1st Division. On July 2, 1863, Sickles insubordinately moved his corps from its assigned defensive position on Cemetery Ridge. This meant Birney's new position was from the Devil's Den, to the Wheatfield, to the Peach Orchard, part of a salient directly in the path of the Confederate assault. It was too long a front for a single division to defend. Assaulted by the divisions of Maj. Gens. John Bell Hood and Lafayette McLaws, Birney's division was demolished. Army commander Meade rushed in reinforcements, but the line could not hold. Birney’s division and the entire III corps were finished as a fighting force.
As Birney watched the few survivors of his division gather about him on Cemetery Ridge, he whispered to one of his officers, "I wish I were already dead". Birney assumed temporary command of the corps after Sickles was grievously wounded by a cannonball, despite having received two minor wounds himself.