Wishing to follow in his father's footsteps, Johnson won permission from General Enoch Crowder to attend the University of California (at Berkeley) where he received his Bachelor of Arts degree (with honors) in 1915 and his Juris Doctor in 1916 (doubling up on courses to graduate in half the time required). Transferring to the Judge Advocate General's Corps (JAG), from May to October 1916 he served under General John J. Pershing in Mexico with the Pancho Villa Expedition. promoted to captain on July 1, 1916, he transferred to the JAG headquarters in Washington, D.C., in October 1916. He was promoted to major on May 15, 1917, and to lieutenant colonel on August 5, 1917. He was named Deputy Provost Marshal General in October 1917, and the same month was named to a Department of War committee on military training (the U.S. had entered World War I on April 6, 1917).