Henry Mainwaring was born in Ightfield, England, second son of Sir George Mainwaring and his wife Ann. His grandfather was Sir William More, Vice-Admiral of Sussex.
Henry Mainwaring was born in Ightfield in Shropshire, second son of Sir George Mainwaring and his wife Ann, the daughter of Sir William More of Loseley Park in Surrey. His maternal grandfather was Sir William More, Vice-Admiral of Sussex.