He took a position as an Army contract physician in 1885, and was stationed at Fort Huachuca, Arizona. Wood participated in last campaign against Geronimo, and was awarded the Medal of Honor in 1898 for carrying dispatches 100 miles through hostile territory and for commanding an infantry detachment whose officers had been lost.
Wood was personal physician to Presidents Grover Cleveland and William McKinley through 1898. It was during this period he developed a friendship with Theodore Roosevelt, then assistant secretary of the navy.
At the outbreak of the Spanish-American War, Wood, with Roosevelt, organized the 1st Volunteer Cavalry regiment, popularly known as the Rough Riders. Wood commanded the regiment in a successful engagement known as the Battle of Las Guasimas, and later led them to a famous victory at Kettle Hill and San Juan Heights as commander of a larger force to which the Rough Riders were attached.
After San Juan, Wood led the 2nd Cavalry Brigade for the rest of the war; he stayed in Cuba after the war and was Military Governor of Santiago in 1898, and of Cuba from 1899-1902. In that capacity he relied on his medical experience to institute improvements to the medical and sanitary conditions in Cuba. He was promoted to Brigadier General shortly before moving to his next assignment.
In 1902 he proceeded to the Philippines, where he served in the capacity of commander of the Philippines Division and later as commander of the Department of the East. He was promoted to major general in 1903, and served as governor of Moro province from 1903-6. During this period he was in charge of several bloody campaigns against Filipino troops, which was to be the beginning of his unpopularity there.
Wood was named Army Chief of Staff in 1910 by President Taft, whom he had met while both were in the Philippines; he remains the only medical officer to have ever held that position. As Chief of Staff, Wood implemented several programs, among which were the forerunner of the Reserve Officer Training Corps program, and the Preparedness Movement, a campaign for universal military training and wartime conscription. The Preparedness Movement plan was scrapped in favor of the Selective Service System, shortly before World War I. He developed the groundwork for the "Mobile Army," thus laying the groundwork for American success in WWI.
In 1914, Wood was replaced as Chief of Staff by John Pershing. With the outbreak of World War I, Wood was recommended by Republicanss, in particular Henry Cabot Lodge, to be the U.S. field commander; however, War Secretary Newton Baker instead appointed Pershing, amid much controversy. During the war, Wood was instead put in charge of the training of the 10th and 89th Infantry divisions, both at Camp Funston.
Wood was an unsuccessful candidate for the Republican presidential nomination in the election of 1920.
He retired from the Army in 1921, and was made Governor General of the Philippines, in which capacity he served from 1921 to 1927. He was noted for his harsh, unpopular policies.
Wood died in Boston, Massachusetts after undergoing surgery for a brain tumor which had resulted from an earlier head injury. He is buried in Arlington National Cemetery.
Camp Leonard Wood in Missouri, now Fort Leonard Wood, home of the U.S. Army Chemical School, was named in his honor, as was the USS Leonard Wood (AP-25/APA-12).