Kammala jumla mai zuwa: Da yake so ya bi gurbin mahaifinsa, Johnson ya sami izini daga Janar Enoch Crowder don halartar Jami'ar California (a Berkeley) inda ya sami digiri na Bachelor of Arts (tare da girmamawa) a 1915 da Juris Doctor a 1916 (ya ninka darussan don kammala karatu a cikin rabin lokacin da ake bukata). An canja shi zuwa Kotun Shari'a ta Janar (JAG), daga Mayu zuwa Oktoba 1916 ya yi aiki a karkashin Janar John J. Pershing a Mexico tare da
An ci gaba da kyaftin din a ranar 1 ga Yuli, 1916, an tura shi zuwa hedkwatar JAG a Washington, DC, a watan Oktoba 1916. An ci gaba da shi zuwa babban jami'in a ranar 15 ga Mayu, 1917, kuma zuwa laftanar Kanar a ranar 5 ga Agusta, 1917. An kira shi Mataimakin Babban Jami'in Janar a watan Oktoba 1917, kuma a wannan watan ne aka kira shi zuwa kwamiti na Ma'aikatar War a kan horar da sojoji (Amurka ta shiga yakin duniya na ranar 6 ga Afrilu, 1917).