United States in World War I
Upon the entry of the United States in World War I, he was appointed acting chaplain with the rank of Lieutenant Junior Grade and assigned to the 11th Marine Regiment in France, where he served until 1919. Rentz attained the rank of Commander in 1924. As chaplain, he served aboard , , , and his final duty station, "Houston". During his military career, Rentz also served at the Marine Barracks in Port Royal, South Carolina, Naval Air Station Pensacola, Florida, and Naval Air Station San Diego, California.