Her first work, "Britain's Royal Families" (published in 1989), was a genealogical overview of the British royal family. She subsequently wrote biographies of Eleanor of Aquitaine, Isabella of France, Katherine Swynford, Elizabeth of York, and the Princes in the Tower. Other focuses have included Henry VIII and his family and England's Medieval Queens. Weir has published historical overviews of the Wars of the Roses and royal weddings, as well as historical fiction novels on English queens, including each wife of Henry VIII.