Mad Jack may refer to:
People
John Byron (British Army officer) (1756–1791), English captain, father of Lord Byron Jack Churchill (1906–1996), British World War II lieutenant colonel who fought armed with a longbow, arrows, and a Scottish broadsword sword, and carried around bagpipes Mad Jack Fuller (1757–1834), English politician, noted for building a number of follies in Sussex Mad Jack Hall (1672–1716), Jacobite leader Charles Howard, 20th Earl of Suffolk (1906–1941), World War II bomb disposal expert John Mytton (1796–1834), English eccentric and rake John Percival (1779–1862), US Navy captain Siegfried Sassoon (1886–1967), English First World War poet, writer and soldier
Fictional characters two of the various Jack O'Lantern (Marvel Comics) villains; the title character of Mad Jack the Pirate, a 1990s cartoon; the boss of the "Frantic Factory" world from the Donkey Kong 64 video game; the embodiment of Samurai Jack's negative emotions given form, from the first and fifth season of the TV show.