John Denis "Jack" Leonard, Baron Leonard, OBE (19 October 1909 – 17 July 1983) was a British Labour Party politician. Leonard was born in Manorhamilton, Ireland, the son of an Irish journalist. He emigrated to the United Kingdom in 1939, where he set up an engineering firm in Cardiff. He was elected to the Cardiff City Council in 1970, and to the South Glamorgan County Council in 1974, serving as its Chairman between 1976 and 1977. He was appointed an OBE in 1976.