William Forster was born in Tottenham. From a very early age, he demonstrated a spiritual frame of mind. After completing is education he followed his father's profession as a land agent. He became an itinerant minister for several years, travelling to many parts of England and Scotland.

William visited Newgate prison with Stephen Grellet. The scenes shocked and sickened them. William, who would have known Elizabeth Fry from birth, contacted her and described the wretchedness of the women at Newgate. Elizabeth Fry gathered together a group of young women Friends and visited the prison. This was the start of Elizabeth Fry's work as a prison reformer.

In October 1816 he married Anna Buxton (Elizabeth Fry's sister). After the marriage they moved to Dorset.