Diamond Allan Ballantyne was born at Samari, New Guinea in 1911, the son of missionary parents. He was educated at Wellington College, New Zealand and the Otago Medical School graduating MB ChB (New Zealand) in 1937. He was appointed a member of the house staff, Napier Hospital, from 1937 to 1939. He enlisted with the New Zealand Medical Corps and proceeded overseas with the first echelon in 1939. He saw action in North Africa, Greece and Crete. He volunteered to remain with his wounded comrades in Crete and he was taken prisoner of war. He was mentioned in dispatches twice. An account of his brave, capable and distinguished services to his wounded colleagues is detailed in one of Brigadier WB (Sandy) Thomas's books, 'Dare to be Free' (1951). He spent the rest of the war in Germany.

He took his discharge in England in 1945, and studied at the Post Graduate Hospital, Hammersmith, the Middlesex Hospital, and later at the Brompton. He studied under and became lifelong friends of Professors Scadding and Bywaters at the Brompton and Hammersmith Hospitals, and the late Professor Beaumont at the Middlesex. He gained MRCP (London) in 1946 and was elected a fellow in 1969. He gained MRACP in 1952, and was elected FRACP in 1958. He was a consulting general physician in Hawkes Bay from 1947, to his retirement from the Memorial Hospital, Hastings in 1976. In his retirement he was acting director of the Hawkes Bay Psychiatric Unit from 1976 to 1981.

He was an erudite and distinguished general physician, widely respected by his colleagues and much loved by both hospital and private patients. He had a lifelong interest in music, having studied this while a medical student at Dunedin. He was an accomplished pianist. His main hobbies were fly fishing for trout and working his small mixed agricultural farm near the Hastings Hospital. He was often seen driving a tractor.

His interest in the Army Medical Corps continued after the war and he was officer commanding the 2nd New Zealand General Hospital with the rank of colonel. He also earned a Merit Award. For a period he was honorary physician to the Queen. He had a life-long interest in the St John's Ambulance and was awarded the Order of St John for distinguished services. He was happily married, and his wife Mrs SJ Ballantyne still lives in Hastings. There were no children. He belonged to an era of specialist general physicians that is fast disappearing.