He was educated at George Watsons College and at Glasgow University, where in 1893 he gained a first- class honors degree in mental philosophy. From there he moved on to teach philosophy for a year at the University College of Wales at Bangor. He then returned to Scotland, was admitted to the Faculty of Advocates, and built up a Glasgow-based commercial and shipping legal practice.
During World War I, he worked with Sir Eric Geddes in organizing the railway network behind the front line in France. After the war, he handled labor relations in the dockyards for the Admiralty.
In March 1920 he became president of the Board of Trade, succeeding Eric Geddes, and in April 1921, chancellor of the exchequer, succeeding Austen Chamberlain.
When Andrew Bonar Law formed a Conservative government in October 1922, replacing the coalition government of David Lloyd George, Horne felt obliged to remain loyal to his coalition associates—particularly to Austen Chamberlain, who had stood down as Conservative leader in response to the removal of Lloyd George. Horne was offered the opportunity of being chancellor of the exchequer again when Stanley Baldwin replaced Bonar Law as prime minister in May 1923, but he refused the offer. He thus became a backbencher in the House of Commons until he retired from Parliament in 1937, when he was raised to the House of Lords as the first Viscount Horne of Slamannan. For the rest of his life he was involved in business as well as politics, becoming a director of the Suez Canal Company, and in 1934, chairman of the Great Western Railway Company. He died in 1940.
Horne’s interest in politics began when he became president of the University Conservative Club. That interest revived in 1910, when he ran twice—unsuccessfully—in the general elections for the seat of Stirling. In December 1918 he won the Glasgow constituency of Hillhead for the Conservatives. He was immediately made minister of labor, and he became one of the few M.P.s to make their first parliamentary speech from the dispatch box on the government side.
He would barely be remembered today were it not for Stanley Baldwins memorable description of him as “that rare thing, a Scots cad.” However, others among his contemporaries considered Horne debonair and something of a high-society person.
Although he never married, he enjoyed the company of smart, fashionable women.