Dunn was born in Scotland and educated at Glasgow University, where he took an arts degree followed by his medical degrees. In 1912 he received his MD for a thesis on the oxydase reactions. During the First World War, Dunn conducted research on nephritis and gas-gangrene, and also studied the effects of pulmonary irritant gases. His reputation was made in the area of renal pathology. In 1919 Shaw Dunn was made professor of pathology at Birmingham University, and moved in 1922 to the same chair at the University of Manchester, and he was also pathologist at the MRI. He remained in Manchester until 1931, when returned to Glasgow.