Private First Class Casillas was a member of Company C, 1st Battalion, 1st Marines, 1st Marine Division. He was Killed in Action while engaged with the enemy during the First Battle for Seoul, South Korea, at the main railroad station on 26 September 1950.

Casillas was awarded the Purple Heart, the Combat Action Ribbon, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.