Sir James Duncan Millar KC MP (5 August 1871 – 10 December 1932) was a Scottish barrister and Liberal, later National Liberal politician.
James Duncan Millar was the son of John Millar, a medical doctor from Edinburgh. He had family connections to Duncan McLaren a former Edinburgh Member of Parliament (MP) and to the famous Liberal John Bright. His maternal grandfather was James Duncan who was a member of The Society of Writers to the Signet the Edinburgh legal association. He was educated at Edinburgh University where he obtained an MA degree and a Bachelor of Laws(LLB). In 1906 he married Ella Forester-Paton of Alloa. They had one son and one daughter. Their son, Ian Alistair Duncan Millar, a Perthshire farmer, stood as Liberal candidate in the Kinross and West Perthshire by-election in 1963 at which Alec Douglas-Home was returned to the House of Commons, coming second to Home.
James Duncan Millar was one of the few men to become a member of both the Scottish and English Bars. He was admitted to the Faculty of Advocates in Edinburgh in 1896 and in the following year was called to the English Bar at the Middle Temple. He obtained a large practice in Scotland and was Senior Advocate Depute from 1913-1916.
He was made a King's Counsel in Scotland in January 1914.