Benson was born on 17 October 1854 at 6 Sussex Square, Paddington, and was educated at Winchester College, before attending New College, Oxford between 1874 and 1877 studying classics and philosophy. As a child he had been taught how to practice metal work on a lathe, but upon leaving Oxford, he was articled to the London architect Basil Champneys. While working for Champney, he met Edward Burne-Jones at a rehearsal of Wagner, and was used by Burnes-Jones as his model for the King in King Cophetua and the Beggar Queen. Burnes-Jones would introduce Benson to William Morris. While completing his architectural studies, he started designing furniture for Morris.
By 1880 he had opened a small workshop in North End Road, Fulham making furniture for Morris & Co. A year later the company employed John Lovegrove, a skilled brass and copper-worker, and Benson moved into the design of metal work including fire screens and fenders, tableware, kettles, urns and light fittings. The success of the company saw them move to a new factory at Eyot Works, St Peter's Square, Hammersmith, a foundry in Chiswick and opening a showroom at Camden Hill Square in Kensington during 1883.
The company would move their showroom to 82 Bond Street, London in 1887, and in their 1899-1900 catalogue the business offered over 800 items that could be ordered. The business also offered commissioned designs, including equipping solicitor J.S. Beal house called Standen with electric lighting, one of the first houses to be fully electrified in Britain. A big commission for Benson was the design and installation of the electric lighting at St. Paul's Cathedral between 1899 and 1900, while he provided the lighting for Siegfried Bing at the Maison de l'Art Nouveau in 1895. Benson would continue to design and manufacture furniture for both Morris & Co and J.S. Henry & Co, and upon William Morris death in 1896 he was appointed as the director for furniture at Morris & Co. Benson would also collaborate with John Henry Dearle on the company's wallpaper designs. W.A.S. Benson & Company became a registered limited company in 1900, with Benson as chairman and Henry Currier Marillier as company secretary. In 1906, the managers of Morris & Co, Frank and Robert Smith approached Benson and Marillier about purchasing the Morris company, which they did and ran both companies jointly until 1908, when Benson concentrated on his own business and Marillier on Morris & Co after personal conflict. During World War I the company switched production to shells for the Ministry of Munitions, and equipment for the launching of torpedoes for the Admiralty. In 1920, Benson sold the business and retired to Castle Corner, a house in Manorbier, Wales with his wife Venetia and died four years later on 5 July.