John or Johnny Duncan may refer to:
Arts and entertainment
John Duncan (painter) (1866–1945), Scottish painter
John Duncan (harpist) (1904–1998), English musician
Johnny Duncan (actor) (1923–2016), American actor
Johnny Duncan (bluegrass musician) (1931–2000), American skiffle and bluegrass musician
Johnny Duncan (country singer) (1938–2006), American country-music singer
John Duncan (American artist) (born 1953), American artist and musician
Big John Duncan (born 1958), Scottish punk musician
Law, government, and politics
John Holt Duncan (1820–1896), American lawyer and judge, co-founder of Beta Theta Pi at Miami University
Sir John Duncan (Australian politician) (1845–1913), South Australian pastoralist and politician
John Duncan (New Zealand politician, born 1848) (1848–1924), New Zealand member of parliament
John D. C. Duncan Jr. (1884–1958), American politician and lawyer
John Edward Duncan (1884–1959), member of the New Zealand Legislative Council
John Duncan Sr. (1919–1988), U.S. representative from Tennessee
Jock Duncan (diplomat) (John Spencer Ritchie Duncan, 1921–2006), British administrator in Sudan and diplomat
John Alton Duncan (1932–2007), Manitoba judge
John Duncan (American government official) (born 1945), official in the U.S. Department of the Treasury
Jimmy Duncan (politician) (John James Duncan Jr., born 1947), U.S. Representative from Tennessee, son of John Duncan Sr.
John Duncan (Canadian politician) (born 1948), member of parliament from British Columbia
John Duncan (diplomat, born 1958), British diplomat
John Duncan (California politician), former council member and current mayor of Coronado, California
Military
John Duncan (British Army officer, born 1870) (1870–1960), British general
Sir John Duncan (British Army officer, born 1872) (1872–1948), British general
Religion
John Dongan (or John Duncan, died 1413), Manx priest in the Isle of Man and Ireland
John Duncan (English writer) (1721–1808), English priest, miscellaneous writer and army chaplain
John Duncan (theologian) (1796–1870), Free Church of Scotland minister, missionary, hebraist and orientalist
John Duncan (English archdeacon) (born 1933), English Anglican priest, archdeacon of Birmingham, 1985–2001
Science, technology and medicine
John Duncan (weaving writer) (fl. 1800–1818), Scottish writer on weaving
John Duncan (botanist) (1794–1881), Scottish weaver and botanist
John Duncan (surgeon) (1839–1899), Scottish surgeon, president of the Royal College of Surgeons of Edinburgh
John Charles Duncan (1882–1967), American astronomer
John Duncan (neuroscientist) (born 1953), British neuroscientist
John S. Duncan (born 1955), British neurologist
Sports
John William Duncan (1885–1963), Welsh field hockey player
Johnny Duncan (footballer) (1896–1966), Scottish football player and manager associated with Leicester City
Art Duncan (John Arthur Duncan, 1891–1975), Canadian ice hockey player
Ross Duncan (John Ross Duncan, born 1944), Australian cricketer
John Duncan (footballer) (1949–2022), Scottish football player and manager
Others
John Shute Duncan (1768–1844), English academic, writer, and museum curator
John Duncan (explorer) (1805–1849), British soldier and explorer in Africa
John Riley Duncan (1850–1911), Texas lawman
John H. Duncan (1855–1929), American architect
John Duncan (historian) (1938–2022), American historian and writer