McClernand fought in the Spanish–American War, serving in the Santiago campaign. He later saw service in the Philippine Insurrection, commanding the 1st Cavalry Regiment. He was promoted to Brigadier General shortly before he retired from active service on December 29, 1912. He was recalled to active duty the next day, serving as President of the Cavalry Board until 1914. He lived in Easton, Pennsylvania, where he wrote "With the Indians and the Buffalo in Montana," for the Cavalry Journal in 1925. He died on February 9, 1926 and was buried in Section 3, Lot 1931-SW of Arlington National Cemetery.