He graduated in medicine from the University of Glasgow in 1934 and specialised in public health, serving as an assistant Medical Officer of Health in Paisley, Portsmouth and Blackburn.
During the second world war he served in the Royal Naval Volunteer Reserve, electing for active service in the Second Submarine Flotilla, where he gained the rank of surgeon commander. From 1963 to 1965 Semple was appointed as honorary physician to the Queen. He was particularly interested in tackling the problems of undiagnosed tuberculosis and poor housing in the city.
He introduced one of the United Kingdom"s first smokeless zones under the Clean Air Acting 1956.
Under his guidance Liverpool became one of the first local authorities to employ mental welfare officers, and he helped to establish an employment unit for adolescents with learning difficulties.