Johnston an haife shi ne kuma ya girma a Ontario, yana karatu a can kafin ya shiga Jami'ar Harvard da kuma jami'o'in Cambridge da Queen. Ya ci gaba da aiki a matsayin farfesa a wasu cibiyoyin sakandare a Kanada, daga bisani ya yi aiki a matsayin shugaban majalisa a Jami'ar Western Ontario, shugaban Jami'ar McGill, da kuma shugaban Jami'ar Waterloo. A lokaci guda, Johnston ya shiga cikin siyasa da aikin jama'a, yana gudanar da muhawarar siyasa da kuma shugabancin kwamitocin a bangarorin tarayya da na larduna, matsayinsa mafi shahara a wannan fannin shine shugabancin binciken da aka yi game da batun Airbus.
Firayim Minista