1792 Started as a clerk in Timothy Bevan and Sons of London, Druggists and Chymists
1795 Elevated to partner
1797 Allen continued in a partnership with his fellow Quaker, Luke Howard. The company became known as Allen and Howard
William Allen became a close associate of Humphry Davy, and his pharmacy was a centre of scientific research in London.
1807 Founding member of the Geological Society
1824 After marrying Charlotte Hanbury he took two of her nephews into the business, Daniel Bell Hanbury (1794-1882) and Cornelius Hanbury (1796-1869) as Allen, Hanburys and Barry
Became a Fellow of the Royal Society
1841 he was founding President of the Pharmaceutical Society.