[5] He was admitted to the Faculty of Advocates in 1897 with a public defence of an assigned Thesis De diversis regulis juris antiqui,[6] and later became King's Counsel in 1912.

[1][7] For a time he wrote articles on conveyancing for Green's Encyclopedia of Scots Law,[5] and was Editor of the quarterly Juridical Review between 1900 and 1907.

[9] Macmillan suffered an illness, and surgery thereon, in 1917, at which time he decided to cease his nascent political career (then in abeyance for the duration of the Great War).

He served as Lord Advocate from February[12] to November 1924,[13] and was sworn of the Privy Council on 16 April that year.

[14] Macmillan was standing counsel for a vast array of clients, that included the Dominion of Canada from 1928, and for the Commonwealth of Australia from 1929.

[13] Macmillan sat as a Law Lord until 1947 except for a brief period at the outbreak of Second World War when he was Minister of Information.

The Ministry of Information was located in the Senate House, University of London, and the Macmillan Hall there is named after him.

[21] He was elected Trustee of the British Museum,[22] and was in 1934 principal proponent and founder of the Stair Society, which was designed "to encourage the study and advance the knowledge of the history of Scots Law by the publication of original documents and by the reprinting and editing of works of sufficient rarity or importance.

"[23] Macmillan led, over the course of a decade to 7 August 1925, the effort to create the National Library of Scotland; the Committee which he chaired was noticed by Alexander Grant, head of McVitie and Price biscuit makers, who donated the bulk of the endowment [24] This happy event culminated with the passage at Westminster of the National Library of Scotland Act 1925 (15 & 16 Geo.