At the onset of the Civil War, McCarren enlisted in Company H of the Irish-American three-months regiment, the 24th Pennsylvania under
Thomas Alfred Smyth. After this three month enlistment he joined the
1st Delaware Infantry. In September 1861, McCarren was promoted to the rank of corporal. He fought in multiple battles including the
Battle of Chantilly,
Battle of Fredericksburg, and most notably, the
Battle of Gettysburg where McCarren earned the Medal of Honor. On the third day of this battle, McCarren captured the colors of the 13th Alabama in a counterattack against
Pickett's Charge. ==Medal of Honor citation==