McLennan was elected a Fellow of the Royal Society in 1915. McLennan delivered the Guthrie lecture to the Physical Society in 1918. With his graduate student, Gordon Merritt Shrum, he built a helium liquefier at the University of Toronto. In 1923, they became the second group of physicists in the world to successfully produce liquid helium, 15 years after Heike Kammerlingh Onnes. In 1926, McLennan was awarded the Royal Society of Canada 's Flavelle Medal and in 1927 a Royal Medal.