Macmillan is Professor Emeritus of the History of Scottish Art at the University of Edinburgh, and a former Curator of the Talbot Rice Gallery.
Macmillan is author of monographs on Scottish and European artists, including Will Maclean, Steven Campbell, Elizabeth Blackadder, Victoria Crowe, and (with Tom Hewlett) of FCB Cadell.
[8] His 2015 critique of intellectual and moral probity in the contemporary art world, entitled The Thought Police, appeared in Treason of the Scholars.
In 2005 he was awarded the Andrew Fletcher of Saltoun Prize for his contribution to Scottish life by the Saltire Society.
In 2018 he was awarded the Sir Walter Scott Medal[11] of the Royal Society of Edinburgh for his outstanding contribution to the appreciation of Scottish Art and its place within the European Tradition.