After being apprenticed as a gardener at Brathay Hall in the English Lake district, he traveled to London to work at the Hugh Low, Enfield Nursery, then onto the Veitch establishment at Coombe Wood.
Purdom became heavily involved with early Union activities particularly promoting the rights of junior workers which eventually led to his unjustified dismissal.
Despite being a bane to the, then, Kew Director, David Prain, the latter recognized the talents of William Purdom and recommended his employee as being very suitable as a plant collector for a joint venture by Veitch and the Arnold Arboretum of Harvard University to the northern provinces of China in 1909.
Over many months in the field, he traveled and collected from as far north as Duolun Nor and Rehe, to Wutaishan, out to Yulin and Yan'an on the fringes of the Ordos Desert, to the peaks of Tabaishan, as well as in the Minshan Mountains and Lamashan near Jone.
He failed in an attempt to reach the monastery of Labrang and the Amnye Machen range due to the hostility of the local lamas.
Following the expedition, Purdom chose to remain in China and in 1916 was appointed an Inspector of Forests to the Chinese Government in Beijing, particularly concerning himself with re-afforestation projects.
In the later capacity he was based in the city of Xinyang on the Beijing to Hankou rail-line working under the eminent Chinese forester, Han An.
Many of the latter, and also extremely interesting photographs that Purdom took whilst in China in 1916-1920 are held at the RBG Edinburgh on behalf of the Lakeland Horticultural Society.