William James Adam Edward Horne was of Scotch parentage and was educated at the George Heriot Hospital School in Edinburgh and at Heriot Watt College in the same city. He became senior demonstrator in applied physics and electrical engineering at the College and was also a member of the first position Battery of the Edinburgh City (Royal Garrison Artillery) Volunteers. In July 1898 he was appointed chief assistant in the physics department of the South African College, Cape Town, under professor J.C. Beattie*, where he taught drawing, as well as wood and iron work. In January the next year he was called up for military duty in the Anglo-Boer War (1899-1902). He served as a second lieutenant in Kitchener's Horse, and was awarded the Queens Medal and clasps. He was an associate member of both the (British) Institution of Civil Engineers and the Institute of Electrical Engineers, and a Fellow of the Royal Scottish Society of Arts. In 1903 he married Winnifred Mary Ellen Murcott-Williams in Cape Town, but they were divorced in 1911. He later married Ellen Sophia Hayward (born Chambers).
By 1908 Horne was both a lecturer in physics at the South African College, and commander of No. 2 Company of the Cape Garrison Artillery, with the rank of captain. In July that year he resigned his posts to become organiser of technical education in the Transvaal Colony. After the formation of the Union of South Africa in 1910 he continued in the same post, in the Education Department of the Transvaal Provincial Administration. He was a founding member of the South African Association for the Advancement of Science in 1902 and later delivered five papers relating mainly to technical education at the association's annual congresses. The papers were published in the association's annual Report: "The trade schools in the Transvaal" (1913), "The relation of high schools to the University Technical College" (1913), "Physical education" (1915), "The movement towards a national system of technical education" (1917), and "The training of fitter-apprentices in the workshops of the Prussian-Hessian State Railways" (1919).