Humphrey would graduate from the United States Military Academy in 1899 as a cavalry officer and was stationed in Cuba. From 1905 to 1906, he served in the Philippines. From 1910 to 1912, Humphrey was a troop commander in the 7th Cavalry. From 1912 to 1916, he was battalion commander in the Philippine Scouts.[2]
From September 1920 to July 1924, Humphrey served at Fort Leavenworth, Kansas. From 1920 to 1921, he was student at the School of the Line. From 1921 to 1922, Humphrey studied at the General Staff School. From 1922 to 1924, he was a cavalry instructor and then chief of the Cavalry Section. From August 1924 to June 1925, Humphrey was a student at the United States Army War College at Washington Barracks, now Fort Lesley J. McNair.[4]
From 1925 to 1929, he was an instructor at the Army War College before being named to command Fort Oglethorpe, Georgia and the 6th Cavalry Regiment until 1931. At Fort Bliss, he was executive officer and chief of staff for the 1st Cavalry Division. In 1933, he returned to the Army War College as assistant commandant of the college and promoted to brigadier general in February 1935. In June, became commanding general of the 1st Cavalry Brigade and post commander at Fort Clark, Texas.[5]