Hugh Macmillan was admitted to the Faculty of Advocates in 1897 and took silk in 1912. In 1924 he became non-political Lord Advocate in the first Labour government of Ramsay Macdonald, and was a Lord of Appeal in Ordinary, 1930-1939, 1941-1947, and Minister of Information, 1939-1940. His talents for chairmanship resulted in appointments to the chair of several important committees and organisations of which the best known was the 'Macmillan Committee', the Treasury Committee on Finance and Industry, 1929-1931.

The special correspondence MSS.25260-25265, includes groups of letters and single letters mostly from persons prominent in public life. MS.25261 contains the largest group of surviving personal letters, those from James Ramsay Macdonald.

MSS.25266-25273 are papers, 1914-1930, relating specifically to the campaign to create a National Library of Scotland, and the early years of the new National Library from its establishment by Act of Parliament in 1925. Macmillan, the lynchpin of the former process, was the most committed supporter of a National Library in the Faculty and its chief negotiator with successive governments.