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. He became a successful advocate, specialising in commercial and shipping cases, and became a
King's Counsel in 1910. He also served as
Examiner in Philosophy (1896–1900) 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. 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. ==Political career==