Educated at Jordanhill School, Arneil went on to attend the University of Glasgow between 1940 and 1945, while also serving as a member of the Home Guard.
He trained in paediatrics under the mentorship of James Holmes Hutchison, and was eventually appointed a consultant at the Royal Hospital for Sick Children.
He quickly developed an interest in nephrology and in 1950 he established a paediatric renal unit at Royal Hospital for Sick Children.
Arneil's unit pioneered the use of cortisone, prednisolone and thiazides in paediatric renal conditions[3] as well as the use of peritoneal dialysis in children with acute kidney injury.
[3] He is credited with the near-eradication of rickets from Glasgow, and when the disease began to reappear in Pakistani migrants he helped to create a successful educational cartoon dubbed into Hindi and Urdu.