Horne was born at Slamannan, Stirlingshire, the son of Reverend Robert Stevenson Horne, the village's Church of Scotland minister, and Mary, daughter of Thomas Lockhead. He was educated at George Watson's College in Edinburgh and the University of Glasgow, where he studied Law^{[citation needed]} and was President of the Students' Representative Council.^{[citation needed]}

Horne then spent a year teaching philosophy at the University College of North Wales, before being elected to the Faculty of Advocates (Scottish Bar) in 1896.^{[citation needed]} He became a successful advocate, specialising in commercial and shipping cases,^{[citation needed]} and became a King's Counsel in 1910. He also served as Examiner in Philosophy (1896–1900)^{[citation needed]} and Rector (1921–1924) at the University of Aberdeen. He was a board member of several companies including directorships of the Suez Canal Company, chairman of the Great Western Railway Company and director of several other companies and banks.^[1]

During the First World War, Horne became Director of Railways on the Western Front with the honorary rank of Lieutenant-Colonel in the Royal Engineers. In 1917 he joined the Admiralty as Assistant Inspector-General of Transportation, becoming Director of Materials and Priority in 1918, and Director of Labour and Third Civil Lord later the same year.^[2]

Having unsuccessfully stood for Stirlingshire in both general elections of 1910,^{[citation needed]} Horne was elected as Member of Parliament (MP) for Glasgow Hillhead in 1918.^[3] He served under David Lloyd George as Minister of Labour between 1919 and 1920, as President of the Board of Trade between 1920 and 1921 and as Chancellor of the Exchequer between 1921 and 1922.^{[citation needed]} It was in that capacity that he was involved in the negotiations leading to the signing of the Anglo-Soviet Trade Agreement. Leonid Krasin pressurised Horne to support the treaty by threatening to cancel orders with textile mills in Yorkshire, as only the mills with Soviet orders were working full-time.^[4] When the treaty was signed, it was the first recognition by Britain of the Russian Soviet Federative Socialist Republic.^{[citation needed]}

When the Lloyd George Coalition Government fell in 1922, Horne refused to join the new government of Bonar Law. Two years later, Stanley Baldwin offered to make Horne Minister of Labour once more, but Horne declined, preferring to concentrate on work in the City.^[1] Although he remained a Member of Parliament until 1937,^[3] he never again held ministerial office. He was appointed a Knight Commander of the Order of the British Empire (KBE) in 1918 for his war services, and raised to Knight Grand Cross (GBE) in the 1920 civilian war honours for his services as Minister of Labour.^{[citation needed]} In 1919, he was also sworn of the Privy Council.^[5] He was ennobled as Viscount Horne of Slamannan, of Slamannan in the County of Stirling, on 9 June 1937.^[6]

Horne, a womanising bachelor, was famously referred to by Baldwin as a "Scots cad", a remark that has stuck.^[7] He died in September 1940, aged 69. The viscountcy became extinct with his death.

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