Bonnin was a member of one of Adelaide's most eminent medical families, being the second youngest of five sons of Dr A J Bonnin. He was educated at St Peter's College and the University of Adelaide Medical School. Immediately after qualification in 1941 he held the post of resident medical officer at the Royal Adelaide Hospital before joining the Army to serve in New Guinea for the rest of the war with the rank of Captain. After his discharge he returned to Adelaide to serve as surgical registrar (1947-48) before coming to the United Kingdom for further postgraduate training. He worked at the Robert Jones and Agnes Hunt Orthopaedic Hospital at Oswestry and while there obtained his Fellowship and his MCh degree in orthopaedics. On returning to Adelaide he served from 1953 to 1956 as surgical superintendent at the Royal Adelaide Hospital, and in 1956 was appointed honorary orthopaedic surgeon to the Adelaide Children's Hospital. It was his habit to reflect his own high standards on his junior colleagues and to disseminate to them any new knowledge or techniques that he had acquired during his travels. Besides his clinical work at the Children's Hospital he was chairman of the honorary staff and on the Board of Management. He also held a number of other honorary appointments, among them being the medical directorship of Somerton's Crippled Children's Home and membership of the State Committee of the Royal Australasian College of Surgeons. Bonnin was a first class oarsman, rowing for his School and University eights and later coaching several university boats; he also represented the university at lacrosse. After returning to Adelaide from England he devoted a considerable amount of his leisure to fishing. He died suddenly from a brain haemorrhage at his home in Adelaide on 24 February 1966, survived by his wife and their two sons and daughter.