Mackenzie was born in Balclutha in 1895. He was the youngest child of Sir Thomas Mackenzie, who was High Commissioner in London and was previously a Liberal politician (and Prime Minister in 1912). Mackenzie Jr. enlisted in the Army in World War I. He was blinded at Chunuk Bair during the Gallipoli campaign and was sent to the No. 2 New Zealand General Hospital at Walton-on-Thames to convalesce. At the hospital he was one of the patients of his sister Mary, who was a member of the Voluntary Aid Detachment here. After recovering he was sent to the live with other blind soldiers in a house in Portland before attending St Dunstan's, the Institute for the Blind Soldiers and Sailors. At the institute he learnt Braille as well as how to type.