On July 1, 1837, Lyon entered the United States Military Academy at West Point. William Tecumseh Sherman, who was a classmate and one year ahead of Lyon, described him as a "lymphatic boy, who didn't seem to have energy enough to make a man." But Lyon did well at the academy. He graduated in 1841, 11th out of a class of 52. Lyon fought in the Second Seminole War in Florida and in the Mexican-American War. During the War with Mexico, he received several promotions for gallantry under fire at the battles of Mexico City, Contreras, and Churubusco. He was then sent to California where developed a reputation as an Indian fighter. Lyon was next sent to Fort Riley in Kansas. There he began to develop strong support for the Union as a result of the political climate developing in the state.