Barbara Palmer, 1st Duchess of Cleveland, mistress of King Charles II; Credit – Wikipedia

• Mistress: a woman who has a continuing, extramarital sexual relationship with one man

This does not purport to be a complete list.

• Rosamund de Clifford, mistress of King Henry II
• Alice Perrers, mistress of King Edward III
• Jane Shore, mistress of King Edward IV
• Bessie Blount, mistress of King Henry VIII
• Mary Boleyn, mistress of King Henry VIII
• Lucy Walter, mistress of King Charles II
• Barbara Palmer, 1st Duchess of Cleveland, mistress of King Charles II
• Louise de Kérouaille, Duchess of Portsmouth, mistress of King Charles II
• Nell Gwyn, mistress of King Charles II
• Arabella Churchill, mistress of King James II
• Catherine Sedley, Countess of Dorchester, mistress of King James II
• Elizabeth Hamilton, Countess of Orkney, mistress of King William III
• Melusine von der Schulenburg, Duchess of Kendal, mistress of King George I
• Henrietta Howard, Countess of Suffolk, mistress of King George II
• Amalie von Wallmoden, Countess of Yarmouth, mistress of King George II
• Maria Fitzherbert, mistress of King George IV
• Frances Villiers, Countess of Jersey, mistress of King George IV
• Mary Robinson, mistress of King George IV
• Isabella Ingram-Seymour-Conway, Marchioness of Hertford, mistress of King George IV
• Elizabeth Conyngham, Marchioness Conyngham, mistress of King George IV
• Dorothea Jordan, mistress of King William IV
• Lillie Langtry, mistress of King Edward VII
• Jennie Jerome, Lady Randolph Churchill, mistress of King Edward VII
• Daisy Greville, Countess of Warwick, mistress of King Edward VII
• Alice Keppel, mistress of King Edward VII
• Freda Dudley Ward, mistress of King Edward VIII while Prince of Wales
• Thelma Furness, Viscountess Furness, mistress of King Edward VIII while Prince of Wales
• Wallis Simpson, Duchess of Windsor, mistress and wife of King Edward VIII, later The Duke of Windsor