Soon after the war he joined Atomic Energy Canada in Chalk River, Ontario, and then in 1949 the chemistry department at the University of Toronto, where he remained until 1982.
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In January 1943 the Cambridge team including Nunn May transferred to the Montreal Laboratory which was building a reactor at Chalk River near Ottawa, Canada.
A meeting in August 1946 with William Valentine Mayneord, while Mayneord was at the Atomic Energy Project at Chalk River, Ontario, contributed to Johns's making a career in medical physics.
In the summers of 1946 and 1947, he worked on the Canadian nuclear power project in Chalk River, Ontario.
It is located at SNOLAB in Canada, and consists of an international collaboration with members from the Université de Montréal, Queen's University, Indiana University South Bend and Czech Technical University in Prague, University of Alberta, Laurentian University and BTI, Chalk River, Ontario.