Achida kutevera tsoka dzababa vake, Johnson akawana mvumo kubva kuna General Enoch Crowder yekuenda kuYunivhesiti yeCalifornia (kuBerkeley) kwaakawana degree rake reBachelor of Arts (nekukudzwa) muna 1915 uye Juris Doctor wake muna 1916 (achiwedzera zvidzidzo kuti apedze muhafu yenguva yaidiwa). Achienda kuJudge Advocate General's Corps (JAG), kubva muna Chivabvu kusvika Gumiguru 1916 akashanda pasi paGeneral John J. Pershing muMexico nePancho Villa Expedition
Akakwidziridzwa kuva kaputeni musi waJuly 1, 1916, akaendeswa kuhofisi huru yeJAG 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).