Private First Class De Leon was a member of Company E, 2nd Battalion, 5th Marines, 1st Provisional Marine Brigade. He was Killed in Action defending his position at the Pusan perimeter in South Korea on 18 August 1950.

De Leon 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.