He was the younger son of eight children, six of whom survived to adulthood, of Sir Watkin Williams-Wynn, 4th Baronet, and, his second wife, Charlotte Grenville.

Among his siblings was elder brothers Sir Watkin Williams-Wynn, 5th Baronet (who married Lady Henrietta Clive, a daughter of Edward Clive, 1st Earl of Powis) and Charles Williams-Wynn, Secretary at War and Chancellor of the Duchy of Lancaster (who married Mary Cunliffe, daughter of Sir Foster Cunliffe, 3rd Baronet).

His sister Henrietta Elizabeth Williams-Wynn, married Thomas Cholmondeley, 1st Baron Delamere.

His father was the only son of Sir Watkin Williams-Wynn, 3rd Baronet and his second wife, Frances Shackerley of Cheshire, and succeeded to the baronetcy (and extensive Wynnstay estates, the largest in North Wales) when only a baby after his father was killed by a fall from his horse while hunting.

[5] Through his eldest daughter Charlotte, he was a grandfather of four, including Countess Helene von Bismarck-Schierstein (1850–1903) (who married Maj. Wilfred Joseph Cripps), and Count Otto Franz Karl von Bismarck-Schierstein (1854–1910).