Trained as a doctor at Edinburgh University, he worked at Birmingham General Hospital from 1779. The story is that one of his patients with dropsy (congestive heart failure) improved remarkably after taking a traditional herbal remedy; William became famous for recognising that the active ingredient in the mixture was contained in the leaves of foxglove. The active ingredient is now known as digitalis, after the plant's scientific name.
He published an early and influential British Flora, of which there were many editions, some posthumus, and carried out pioneering work into the identificaton of fungi.
He is buried in Edgbaston old church, next to the hall. His memorial stone, now moved inside the church, has foxgloves carved upon it to commemorate his discovery. He is remebered by the Moonstones, also in Birmingham.