His father, a Pennsylvanian Quaker had returned to Britain in 1777 during Philadelphia's worst period in the American War of Independence and settled at Higham Lodge, Walthamstow, Essex, UK.

William's letters to his daughter Suzanna are held by the Library Company of Philadelphia and stored at the Historical Society of Pennsylvania.

Dillwyn was also one of the founders of the Royal Institution of South Wales and its first President, and in 1840 he published a short history of Swansea.

His granddaughters by his son Lewis were the novelist and industrialist Amy Dillwyn, and lepidopterist Mary De la Beche Nicholl.

[7] Taxa named by Dillwyn include: [[Category:Whig (British political party) MPs for Welsh constituencies]]