What is the surname of Alan Turing's father?

Alan Turing’s father had the surname Turing. His full name was Julius Mathison Turing, so “Turing” is the correct surname.

Julius Turing worked in the Indian Civil Service of the British Raj. Alan was born in Maida Vale, London, on 23 June 1912 while Julius was on leave from his post in India. Although Alan and his elder brother were raised largely in Britain, their family’s life was shaped by Julius’s civil-service assignments and travel between Britain and India.

The family name can be confused with names from Turing’s maternal side. Alan’s mother was Ethel Sara Turing, whose family included Edward Waller Stoney, chief engineer of the Madras Railways. “Stoney” therefore belongs to his maternal ancestry, not his father’s surname. Ward and Mathison are not the answer: Mathison was Julius’s middle name and part of Alan’s own full name as well.

The surname also became famous through Alan’s work. The “Turing machine” is a formal model of computation, while the Turing test is associated with questions about machine intelligence. Both names ultimately derive from the family surname carried by his father.