Milne was born in Hull, Yorks, England. He was a fellow of Trinity College, Cambridge, 1919–25, being assistant director of the solar physics observatory. 1920–24, mathematical lecturer at Trinity, 1924–25, and university lecturer in astrophysics, 1922–25. He was Beyer professor of applied mathematics, Victoria University of Manchester, 1924–28, before his appointment to the Rouse Ball chair of mathematics and to a fellowship at Wadham College, Oxford, in 1928. Milne's earlier work was in mathematical astrophysics, for which he was awarded the Gold Medal of the Royal Astronomical Society in 1935. From 1932 he also worked on the problem of the "expanding universe" and in Relativity, Gravitation, and World-Structure (1935), proposed an alternative to Albert Einstein's general relativity theory. His later work, concerned with the interior structure of stars, aroused controversy. Milne received the Royal Society's Royal Medal in 1941, and was president of the Royal Astronomical Society, 1943–45.
John Milne of Urquhart, Morayshire, born in 1659, was the father of Harry Milne (b.1695), 1st Laird of Chapelton, Forfar.
However, the view that the name Milne is derived from the old English 'myln', in turn derived from the Latin 'molina', meaning 'mill', and thus referring to people living 'at or near a mill' is open to question, although it is an obvious and apparently plausible explanation.
The arms of the Milne and Molyneux families, if I may refer to them as such, are identical, except that the tinctures (colours) are reversed, as illustrated; this is a recognized form of differencing.