On 12 January 1887, Cronin was commissioned as a Second Lieutenant of the Infantry and served on the United States frontier from 1887–1893. After his frontier duty, Cronin became an instructor at West Point from 1893–1897. With the outbreak of the Spanish–American War, Cronin became Regimental Adjutant of the 25th Infantry and served in the Santiago Campaign of 1898; during his time in Cuba, Cronin was also a member of the Sanitary Corps. In the subsequent Philippine–American War, Cronin served as Colonel of the United States Volunteers and later became Assistant Chief of the Philippine Constabulary from 1915–1917. Following the United States' entry into World War I, Cronin was promoted to Brigadier General of the National Army on 5 August 1917. He became commander of the 163rd Infantry Brigade at Camp Gordon, serving in France as part of the American Expeditionary Force. Cronin later retired in 1925.