Pedzisa mutsara unotevera: Achida kutevera tsoka dzababa vake, Johnson akawana mvumo kubva kuna General Enoch Crowder yekuenda kuYunivhesiti yeCalifornia (kuBerkeley) kwaakawana Bachelor of Arts degree (nekukudzwa) muna 1915 uye Juris Doctor wake muna 1916 (achiwedzera zvidzidzo kuti apedze muhafu yenguva inodiwa). Achichinjirwa kuJudge Advocate General's Corps (JAG), kubva muna Chivabvu kusvika Gumiguru 1916 akashanda pasi paGeneral John J. Pershing muMexico ne
Pancho Villa Expedition. akasimudzirwa kuva kaputeni musi waJuly 1, 1916, akaendeswa kuJAG dzimbahwe muWashington, D.C., muna October 1916. Akakwidziridzwa kuva mukuru musi waMay 15, 1917, uye kuva lieutenant colonel musi waAugust 5, 1917. Akapiwa zita rokuti Deputy Provost Marshal General muna October 1917, uye mwedzi iwoyo wakagadzwa kuDhipatimendi reHondo komiti yekudzidziswa kwemauto (U.S. yakanga yapinda muHondo Yenyika I musi waApril 6, 1917).