John Day, born in or before 19th Century

• John Day (architect), nineteenth-century Irish architect from County Wexford
• John Day (botanist) (1824–1888), Victorian orchid collector and illustrator
• John Day (carpenter) (died 1774), first recorded death in a submarine
• John Day (priest) (1802–1879), clergyman in the Church of Ireland
• John Day (cricketer, born 1812) (1812–?), English cricketer
• John Day (cricketer, born 1881) (1881–1949), English cricketer
• John Day (dramatist) (1574–c. 1638), English dramatist
• John Day (horseman) (1819–1883), English jockey and trainer
• John Day (judge) (1797–1859), Liberian politician and judge
• John Day (merchant) (fl. 1497–98), English merchant, author of a letter referring to existence of lost book Inventio Fortunata
• John Day (Nova Scotia legislator) (died 1775), merchant and politician in Nova Scotia
• John Day (printer) (c. 1522–1584), English Protestant printer, also known as John Daye
• John Day (trapper) (c. 1770–1820), American hunter and trapper
• John Day Jr. (died 1792), soldier and political figure in Nova Scotia
• John B. Day (1847–1925), manager of the New York Giants in 1899
• John Barham Day (1793–1860), English jockey and trainer
• John Charles Day (1826–1908), English judge
• John Medway Day (1838–1905), Australian journalist and activist
• John Other Day, Native American who sought peace between Indian tribes and white settlers

John Day, born in or after 20th Century

• John Day (Australian politician) (born 1955), Western Australian politician
• John Day (computer scientist) (born 1947), ARPANET pioneer and early RFC contributor
• John Day (Indiana politician) (born 1937), Democratic member of the Indiana House of Representatives
• John Day (Old Testament scholar) (born 1948), professor of Old Testament Studies
• John Day (painter) (1932–1982), American painter, abstract surrealist, studied under Josef Albers at Yale School of Art