He enrolled in science at the University of Edinburgh in 1914, but was commissioned in 1915 as an officer in the British Army in World War I and fought at Gallipoli, Egypt and France.

In 1922 he resigned and travelled to Australia, where he bought a dairy farm near Launching Place with a friend from school, Walter McEwen.

[1] In 1927 Dedman stood unsuccessfully as the Country Party candidate for Upper Yarra in the Victorian Legislative Assembly.

[1] At the end of World War II, he became Minister in charge of the Council for Scientific and Industrial Research and Minister for Postwar Reconstruction in the Chifley government, where he was responsible for promoting full-employment, retraining ex-service personnel and attempting to rebuild the national economy.

[1] He was to have the John Dedman Parkway in Canberra named after him, but the road plans were renamed prior to completion as the Gungahlin Drive Extension.