Stuart Douglas Allen was born in Sydney in February 1900. His father was William John Allen, horticulturist, and his mother Annie Douglas Robertson whose father William Whitehead Robertson was an importer. He was educated at Sydney Grammar School and graduated MB ChM from the University of Sydney in 1924. He was a student of Sydney Hospital.
Following qualification he obtained a post as honorary assistant physician at Mater Misericordiae General Hospital which he held from 1935 to 1939, and as honorary assistant physician at Royal North Shore Hospital (RNSH) from 1940 to 1941. From 1941 to 1946 he served in the second AIF attaining the rank of major. He had seen previous service in the first AIF in 1918 prior to his qualification in medicine. After the war he became honorary physician to the Mater Misericordiae General Hospital in 1946 and was honorary physician at RNSH from 1946 to 1950. In 1950 he was made honorary consulting physician at RNSH.
His principal publication was on tropical and anhidrotic asthenia with JP O'Brien in 1944. In his submission to the College in 1968 he listed his chief interests as being 'rural interests'. In 1930 he married Meda, the daughter of William Charles Hadley Lipman, a company director. There were no children.