During the Mexican–American War, Leonard Ross volunteered in the 4th Illinois Volunteer Infantry Regiment, enlisting in June 1846. He was promoted from the rank of private to first lieutenant on September 4, 1846, by the commanding officer, Captain Lewis W. Ross, his older brother. In a letter to his wife, Captain Ross remarked that Leonard "enjoys his honors very well &amp; will make a very good officer. He lacks dignaty [sic] which I am imparting to him as fast as possible from the surplus which I had of the article on hand." Leonard Ross subsequently played prominent roles in a number of battles, including the Siege of Veracruz and the Battle of Cerro Gordo. According to a note on a muster roll signed by Captain Ross and dated April 30, 1847, Lieutenant Ross had sent in his resignation by that date and had already returned home from the war. However, it was also noted by Captain Ross that Lieutenant Ross' resignation was not accepted, so he was officially considered absent without leave, although there were apparently no adverse consequences. An extensive account of Leonard Ross' service during the Mexican-American War has been provided by the State Historical Society of Iowa.