Kammala jumla mai zuwa: Johnston ya fara aiki a cikin Royal Horse Guards kuma ya kasance tare da rundunar a
Yaƙin Dettingen a 1743 da Yaƙin Fontenoy a 1745. Ya zama Manjo a cikin rundunar a ranar 29 ga Nuwamba, 1750, an ba shi mukamin Laftanar-Colonel na Royal Horse Guards a ranar 17 ga Disamba, 1754 kuma ya umurci rundunar a Jamus a cikin alkawura da yawa a lokacin Yakin Shekaru Bakwai.