He studied in Rome, Lille and Oxford and was ordained a priest on 15 April 1928 in Ottawa by Cardinal Felix-Raymond-Marie Rouleau and was then sent on a mission to Japan.
In 1941, he was forced to leave Japan for political reasons, he returned to Canada on 16 January 1941 and was as appointed Titular Bishop of Calydon.
On 26 November 1942 he received the appointment as Apostolic Administrator of the erstwhile Diocese of Gravelbourg in Saskatchewan, Canada.
This was followed on 20 July 1953 by his appointment as Archbishop and Metropolitan of Ottawa, on 17 September 1953 he was officially inaugurated to his new office.
Archbishop Lemieux was a participant in all sessions of the Second Vatican Council and was consultant to the "Preparatory Commission for the bishops and dioceses."