Thomas was a rifle company platoon sergeant with Third Platoon, E Company, 2nd Battalion, 28th Marines. On February 19, his unit landed with the first wave of Marines on the southern beach of Iwo Jima towards Mount Suribachi. On February 21, Thomas took over the command of the Third Platoon from his platoon commander who was wounded. Thomas and his men successfully assaulted a heavily fortified hostile sector at the base of Mount Suribachi. Thomas directed tank fire while under fire during the Marine assaults on the Japanese, and combined with his other actions that day, this would eventually contribute to the capture of the mountain on February 23 by Thomas and his platoon. He was posthumously awarded the Navy Cross for extraordinary heroism on February 21.