Sir John Cunningham McLennan, KBE, FRS, FRSC, physicist (born 14 April 1867 in Ingersoll, ON; died 9 October 1935 in Paris, France). After graduating in physics from the University of Toronto in 1892, McLennan worked as a demonstrator and in 1898 went to the Cavendish Laboratory in Cambridge, England. In 1900 he received the first doctorate in physics from the University of Toronto. He spent his career at that university, bringing his physics laboratory to the forefront of research in radioactivity, spectroscopy and low-temperature physics.

During the First World War, McLennan was scientific adviser to the British Admiralty. In 1915, he was elected to the Royal Society of London (receiving its gold medal in 1928) and in 1917 received the Order of the British Empire. After the war he returned to the University of Toronto but spent each summer in Britain, becoming president of the scientific section of the British Association for the Advancement of Science in 1923.

President of the Royal Canadian Institute in 1916, he helped found the National Research Council and was a member of the Ontario Research Foundation (established 1928). He resigned from the University of Toronto in 1932 and moved to England where he continued his pioneering research in the use of radium to treat cancer. He was knighted in 1935.