After graduating from Tulane University, Lewis enrolled in the United States Military Academy on September 1, 1875, after securing his appointment. He graduated as part of the Class of 1879 on June 13 as a Second Lieutenant of the 2nd Cavalry Regiment which was stationed at the Northwest. He was initially stationed at Fort Custer before being transferred to Fort Walla Walla for a long time before finally being transferred to the Boise Barracks until June 1890 when he was transferred to Fort Lowell, the Jefferson Barracks Military Post and Fort Wingate. While at Fort Wingate, he commanded the Navajo Indian Troop before being transferred to Fort Riley from October 14, 1896, to April 19, 1898.[1][2]
Spanish–American War
During the Spanish–American War, Lewis continued to command the 2nd Cavalry Regiment as part of the Fifth Army Corps. The regiment arrived at Tampa Bay and was sent to Daiquirí, Oriente Province on June 23, 1898. He led the regiment at the Battle of San Juan Hill and the Siege of Santiago and was one of the first to receive news of the Spanish surrender from July 19 to August 16. After the war, he was sent to Montauk, New York, in August 1898 before being transferred to Huntsville, Alabama, and returned to Cuba in February 1899 to command Companies E, F and G of the 2nd Cavalry Regiment.[1][2][3]
Progressive Era and World War I
He was later sent to Fort Myer and was detailed at the General Recruiting Service. Lewis also returned to Fort Riley and then transferred to Fort Sheridan. Lewis was then made a member of the Cavalry Examining Board from August 1906 to July 1909 while being given command of the First Squadron of the 13th Cavalry Regiment from July 27 to September 6, 1907, and participated in the Moro Rebellion. Before being placed on the reserves, Lewis was the adjutant-general of the Southern Department of San Antonio, Texas, and adjutant of the 1st Cavalry Brigade.[1][2]