Williams-Wynn was the eldest son of the second marriage of his father, Sir Watkin Williams-Wynn, 3rd Baronet, to Frances Shackerley of Cheshire. He was a baby when his father was killed by a fall from his horse while out hunting, and he inherited the extensive Wynnstay estates, the largest in North Wales. These straddled at least five Welsh counties and extended into Shropshire in England, and yielded an estimated rental income of £20,000 – a very substantial sum at the time, whose spending he tackled with enthusiasm and considerable success.