Postlethwaite lived in West Itchenor, West Sussex before moving near Bishop's Castle, Shropshire. He began a relationship with Jacqueline "Jacqui" Morrish, a former BBC producer, in 1987; the couple married in 2003 at St Nicholas' Church, Itchenor. They had two children, William John "Billy" (born 1989), and Lily Kathleen (born 1996). Billy is an actor who trained at the London Academy of Music and Dramatic Art, and has most notably appeared in the TV series "Holby City", "Game of Thrones" and "Chernobyl".