New Zealand dentist, horticulturist and amateur botanist. Born to Scottish parents in Dunedin, Thomson was apprenticed to a dentist and after studying dentistry in Pennsylvania and Scotland he returned to New Zealand, living at Halfway Bush, Dunedin, from the early 1900s, where he enthusiastically planted native species in his garden. He helped to form the New Zealand Dental Association at this time.

From about 1910 Thomson also began making excursions to collect plants, travelling to the Garvie Mountains that year with his friends D.L. Poppelwell and J. Speden. They returned there in 1913, and in 1915 visited the Eyre Mountains, where Thomson found a mountain daisy that was named after him by Thomas Cheeseman: Celmisia thomsonii. Over the next few years Thomson collected on Long Island, at Martins Bay and Mount Tennyson. He served in Egypt during the First World War, and afterwards married Mabel Pullen in 1919.

Thomson continued to collect plants in the 1920s, sending material to Cheeseman and to Donald Petrie, who both described novelties found by him. After their deaths he sent specimens to Leonard Cockayne. His garden at Halfway Bush also gained acclaim in this decade, drawing visitors from far and wide to see its native plantings, including Olearias, Senecios, Euphrasia and Nothofagus. Thomson was especially interested in mistletoes and mountain daisies, both of which he cultivated (or attempted to) and wrote articles on. Thomson retired from his dental practice in 1940 and died ten years later. His younger brother, Jack Thomson, was also an amateur botanist. (Neither were related to the Dunedin botanist George Malcolm Thomson (1848-1933).)

Sources:
E.J. Godley, 1995, "Biographical Notes (19): William Alexander Thomson (1876-1950)", New Zealand Botanical Society Newsletter, 41(September): 18-20.