Colonel William Montagu, 5th Duke of Manchester (21 October 1771 – 18 March 1843), styled Viscount Mandeville until 1788, was a British peer, soldier, colonial administrator and politician.

After his return to Britain Manchester served as Postmaster General between 1827 and 1830 (succeeding his younger brother Lord Frederick Montagu).

According to The Complete Peerage, "it is mentioned in the Memoirs of a Highland Lady, under date 1812, that 'the Duchess had left home years before with one of her footmen.'

Lady Jerningham wrote, 6 September 1813: 'the Duchess of Manchester is finally parted from her husband, her conduct becoming most notoriously bad'".

Manchester survived his wife by fifteen years and died in Rome, Italy, in March 1843, at age 71.