Born in Larne in Northern Ireland, Magill qualified in medicine from Queen’s University, Belfast, in 1913. After resident medical appointments he joined the Royal Army Medical Corps and served in France during the First World War.
In 1919, pending demobilisation, Magill was posted to Queen Mary’s Hospital for Facial and Jaw Injuries in Sidcup. This was a special military hospital with an international surgical staff undertaking reconstructive surgery of war wounds including the pioneer plastic surgeon Major (later Sir) Harold Gillies. Magill’s task was to administer anaesthesia, even though he had very little practical experience in this. Generally, anaesthetics in this period were given using mask covering the nose and mouth, making operations very difficult for surgeon operating on the face and jaws. With Stanley Rowbotham, Magill developed wide-bore intubation and this procedure has since become essential practice for major operations of all categories.
Magill went on to become the leading exponent of anaesthesia for the developing specialty of thoracic surgery and was an initiator and examiner for the Diploma in Anaesthetics in 1935. He anaesthetised King George VI for two major operations in the 1940s and was appointed a Commander of the Victorian Order in 1946 and knighted in 1960. He died, greatly honoured in 1986, aged 98.