After teaching for two years at Ramsgate College, he returned to Aberdeen and thence to the University of Edinburgh, earning an MB ChM in 1883 with highest honours.

In 1905 he became Director of the Royal Botanic Gardens, Kew,[5] a post he held until 1922 when he was succeeded by Sir Arthur William Hill.

William Purdom was ferocious as the union representative, bringing the newspapers of the time, the Kew Guild and leading politicians into the fray.

Prain, who came from a humble background himself, was aware that his workers' grievances were well justified and went out of his way to find alternative positions in private employ for all those affected.

Despite being a bane to the Kew Director, the latter clearly recognized the talents of William Purdom and recommended his employee as a plant collector for a joint venture by Harry Veitch and the Arnold Arboretum of Harvard University to the northern provinces of China in 1909.

No sooner had Purdom's dismissal been finalized, than that same establishment arranged for the British administration, including the Legation in Beijing, to give him all assistance.

[1][7] In 1907 he was awarded an honorary doctorate of philosophy at the Linnaeus' tercentenary in Uppsala, Sweden, and became a member of the Royal Swedish Academy of Sciences in 1912.

They had one son, Theodore Prain (1888-1914), who was killed in the first weeks of the First World War whilst serving as a lieutenant in the Leicestershire Regiment.