Amin ya shiga King's African Rifles (KAR) na British Colonial Army a shekara ta 1946 a matsayin mataimakin mai dafa abinci. A cikin rayuwarsa ta baya ya yi ikirarin karya cewa ya yi aiki a cikin Burma Campaign na yakin duniya na biyu. An tura shi zuwa Kenya don aikin soja a matsayin mai zaman kansa a 1947, kuma ya yi aiki a cikin 21st KAR infantry battalion a Gilgil, Kenya Colony har zuwa 1949. A wannan shekarar, an tura rundunarsa zuwa arewacin Kenya don yaƙi da 'yan tawayen Somaliya a cikin Shifta War. A shekara ta 1952, an tura brigade dinsa a kan' yan tawayen Mau Mau a Kenya. An daukaka shi zuwa corporal a wannan shekarar, sannan zuwa sergeant a shekara ta 1953. A shekara ta 1959, Amin ya ci gaba, mafi girman matsayi ga wani baƙar fata na Afirka a cikin sojojin mulkin mallaka na Birtaniya a wannan lokacin. Amin ya ci gaba zuwa Uganda a shekara ta 1961, an daukaka shi zuwa babban jami'in soja a Uganda, ya zama babban jami'in soja a lokacin yakin basasa na sojojin Ingila a shekarar 1955, ya zama babban jami'in kwando a lokacin da aka nada
Shekara ta 1962