Scholar of English illuminated manuscripts and Keeper of Manuscripts, British Museum,1944-47. Millar's parents were George Millar (d. 1889) and Edith Antsey (Millar). He was the nephew of Thomas Anstey Guthrie (1856-1934), a cartoonist for Punch. Millar was educated at Charterhouse and Corpus Christi College, Oxford. His uncle's friends included a bicycled riding companion, the manuscript scholar Montague Rhodes James (1862-1936). Guthrie, who went by the name of Antsey, convinced James to give his nephew an appointment at the British Museum, Department of Manuscripts in 1912.