In 1946 Williams-Wynn was promoted Lieutenant Colonel and given command of the 361st (Carnarvonshire and Denbigh Yeomanry) Medium Regiment of the Royal Artillery (Territorials), retaining it until 1952, after which he was Honorary Colonel of the Regiment until October 1957. In 1947, he was appointed a Deputy Lieutenant for Denbighshire. On 23 November 1951, on the death of his father, he inherited the Williams-Wynn Baronetcy, and was High Sheriff of Denbighshire in 1954. He was also Master of the Flint and Denbigh Foxhounds from 1946 to 1961 and became Joint Master of his own Sir W. W. Wynn's Hounds in 1957. From 1961 to 1970 Williams-Wynn was Liaison Officer to the Ministry of Agriculture for North Wales, and from 1963 to 1966 a member of the Nature Conservancy for Wales. He was a president of the National Eisteddfod of Wales, Vice-Lieutenant of Denbighshire from 1957 to 1966, then Lord Lieutenant from 1966 to 1974. With a reorganisation of Welsh counties in that year, he served as Vice-Lieutenant of Clwyd from 1974 to 1976 and as Lord Lieutenant of Clwyd until 1979, when he retired. At the time of his death on 13 May 1988, Williams-Wynn was living at Llangedwyn Hall, Powys, in the border country near Oswestry, Shropshire. The ruins of Owain Glyndŵr's Sycharth stand nearby. An obituary in "The Times" said "He was a countryman to his bones. From his estate at Llangedwyn, South Clwyd, he exercised his wide agricultural and conservationist interests". In his Will, he left an estate valued at £736,062. In 1939, Williams-Wynn married firstly, at Holy Trinity, Brompton, Margaret Jean, the daughter of Colonel William Alleyne Macbean, late Royal Artillery, and the Hon. Mrs Gerald Scarlett, step-daughter of Major General Gerald Scarlett. They were married by William Havard, Bishop of St Asaph. They had two sons, of whom David Watkin was heir to the title and estates. His wife died in 1961, and in 1968 Williams-Wynn married secondly Gabrielle Haden Matheson, the daughter of Herbert Alexander Caffin.