Sir Denis was born in 1907 in Wellington. He was educated at Waitaki Boys High School and Cambridge University before being called to the Bar in England in 1929.
He returned to New Zealand in 1930, practising as barrister and solicitor in Wellington. He was a partner in the Wellington law firm of Bell Gully from 1936 to 1968. During the Second World War he was a Lieutenant-Colonel in the 2 NZEF (1939-44), for which service he received the OBE.
He was President of the Wellington District Law Society in 1951, President of the New Zealand Law Society for six years (1962–1968) and Vice-President of the Law Society of Asia and the Pacific in 1966.
He was appointed as High Commissioner for New Zealand in Britain and Ambassador to Ireland in 1968. He held these posts until his appointment in 1972 as Governor-General of New Zealand.
A gifted cricketer who represented New Zealand at international level, Sir Denis was President of the New Zealand Cricket Council (1959-62).
Sir Denis died in 1984. His wife, Lady June Blundell ONZ, QSO, died on 31 October 2012.