On November 26th the 138th marched from Brandy Station and crossed the Rapidan River. The next day the Confederate army, who had retreated from Brandy Station was encountered around noon, by this time the Army of the Potomac was already engaged in the fighting. Keifer's Brigade, which included the 138th, was ordered to form at the left of the Union center. The 138th was placed at the extreme left of this line in an open field. Around three in the afternoon, the Stonewall Brigade will mass upon the 138th defensive line. The Confederates will surge a total of two times, each time they will be shot down and pushed back. "I cannot pause to describe the constant roar of musketry and the terrific fire," Sergeant Major Henry Grossman. Later in the fight, Colonel M'Clennan will fall with a wound to the foot. Nicholas G. Wilson—1st Sergeant of Company G, will have his knapsack blown off his back after a rebel bullet pierces it and ignites the 40 rounds of ammunition he stored in it.
The Battle of Locust Grove will end with the second repulsement of the Confederates. Moral will greatly improve for the regiment and veterans will look back on this battle with a great sense of accomplishment. During the night the regiment will be pulled from the line and resupplied. In total three officers are killed, 52 enlisted men are killed and one officer wounded. The number of enlisted wounded is not clear. The Confederates will retreat during the night. Twice the regiment was ordered to prepare to give chase but the order to advance was never given. Winter quarters will be set up at Brandy Station.
Mine Run Campaign Battle Map 
Locust Grove/Payne's Farm Battlefield