John or Jack Leonard may refer to:
- John Leonard (songwriter), early 19th century radical poet and songwriter, probably born in Gateshead
- John Leonard (critic) (1939–2008), American literary, television, film, and cultural critic
- John Leonard (poet) (born 1965), Australian poet
- John Leonard (Gaelic footballer) (born 1976), Gaelic footballer
- John Leonard (canoeist) (born 1948), New Zealand sprint canoeist
- John Leonard (judge) (1926–2002), British judge
- Johnny Leonard (1903–1995), Australian rules footballer
- John B. Leonard (1864–1945), American civil engineer
- John J. Leonard, American roboticist and professor at MIT
- Jack Leonard (footballer) (1876–?), English footballer
- Jack Leonard (hurler) (1873–1938), Irish hurler
- Jack E. Leonard (1910–1973), American comedian and actor
- John E. Leonard (1845–1878), United States representative from Louisiana
- John Leonardi (1541–1609), Catholic saint whose feast day is on 9 October
- John Leonard (bishop) (1829–1908), Irish bishop
- John W. Leonard (1890–1974), US Army general
- John Leonard, Baron Leonard (1909–1983), British Labour Party politician
- John Leonard (ice hockey), (born 1998), American ice hockey player
- John Leonard (architect), English architect
- J. Paul Leonard (1901–1995), American educator and university president
See also
- John Lennard (disambiguation)