The regiment reached the battlefield at Gettysburg with the 6th Corps on the afternoon of July 2, 1863. Here it took up a position which it held, checking the enemy in every attempt to penetrate the Union lines.
The 139th went into action just as the U. Regulars were being driven back from the Wheatfield and helped defend the left flank of the Union army. After the Regulars withdrew through their line, the 139th Pennsylvania counter-charged, halting the Confederate attack.
Colonel Collier was wounded early in the morning on July 3rd, and Lieutenant Colonel William H. Moody then took command of the regiment, which advanced after Pickett's Charge, clearing the area along the Wheatfield Road and recovering a cannon and three caissons of the Ninth Massachusetts Battery which had been lost on July 2nd. At Gettysburg, the unit had 511 troops engaged with 1 killed and 19 wounded. This secondary monument was dedicated in 1886 and was originally placed at the current location of the main monument at the Weickert Farm. When the newer, main monument was placed in 1889, this original monument was relocated to the position reached by the regiment in its advance on July 3, 1863; it is now located on the east side of Sickles Avenue.