In 1820 William undertook a mission to America on behalf of the Society of Friends. This lasted for five years during which time he worked to prevent a rift that was appearing among American Friends. The separation went ahead, but William was considered to have done good service. He had a calm and peaceful tone which suited him to conciliatory work.

William made two further visits to America. One of these was due to a threatened split among Friends in Indiana over the slavery question. His efforts were successful. William was a strong advocate against slavery.