Collins was commissioned as a second lieutenant into the Infantry Branch of the United States Army and was assigned as a platoon and later company commander with the 22nd Infantry Regiment. He was promoted to first lieutenant in May 1917, and temporary captain in August. He attended the United States Army Infantry School of Arms at Fort Sill, Oklahoma and served with the regiment at various locations between 1917 and 1919. He was promoted to captain in June 1918, and to temporary major in September, and took command of the 3rd Battalion, 22nd Infantry Regiment the following month. World War I came to an end soon afterwards, on November 11, 1918. Unable to fight overseas during the war, Collins commanded the 3rd Battalion, 18th Infantry Regiment in France in June 1919, and was assistant chief of staff, as a G-3 staff officer with the American Forces in Germany from 1920–1921. During this time, Collins served in the Army of Occupation in Germany.