Born in Ingersoll, Ontario, the son of David McLennan and Barbara Cunningham, he was the director of the physics laboratory at the University of Toronto from 1906 until 1932.

[2] 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.

He died in 1935 near Abbeville in France on a train from Paris to London[2] of a heart attack.