Hugh Pattison Macmillan, Baron Macmillan, was born on February 20, 1873, in Glasgow, Scotland.

He was a Scottish advocate, judge, parliamentarian, and civil servant. He was educated at Collegiate School, Greenock (1878), Edinburgh University (M.A., 1893), and Glasgow University (LL.B, 1896). He was admitted to the Faculty of Advocates in 1897 and became King's Counsel in 1912. He wrote articles on conveyancing for Green's Encyclopedia of Scots Law and was editor of the quarterly Juridical Review between 1900 and 1907. During the First World War, Macmillan served as Assistant Director of Intelligence for the Ministry of Information. In 1923, he was elected a Fellow of the Royal Society of Edinburgh. He was sworn to the Privy Council in April 1924 and served as Lord Advocate from February to November 1924. In London, Macmillan enjoyed a varied practice in the House of Lords and the Privy Council, as well as before parliamentary committees. He was appointed standing counsel for Canada (1928) and Australia (1929). In 1930, Macmillan was made a Lord of Appeal in Ordinary with the customary life peerage as Baron Macmillan of Aberfeldy in the County of Perth, a post he held until 1947. He held several chairmanships, including the Committee on Finance and Industry (1929–1931), the Canadian Royal Commission on Banking and Currency (1933), the Pilgrim Trust (1935-1952), the Political Honours Committee (1935-1952), the Court of the University of London (1929-1943), and the BBC Advisory Council (1936-1946). He was elected Trustee of the British Museum and was a principal proponent and founder of the Stair Society in 1934. Macmillan received an honorary degree of LL.D. from Edinburgh, London, St. Andrews, McGill, Queens, Dalhousie, and Columbia universities. In 1924, he was elected the first Honorary Bencher of Inner Temple and was awarded the Royal Victorian Order in 1937.

In 1901, he married Elizabeth Katherine Grace Marshall (1873–1967). He died on September 5, 1952, in Ewhurst, Surrey, England.