[1] He was the eldest son of Col. Robert Edward Myddelton (1866–1949)[2] and Lady Violet Nevill (1866–1910) who married in 1898.
[3] From his parents marriage, he had a sister, Idina Joan Myddelton (the wife of John Charles Trueman Mills, and later, Roland Cubitt, 3rd Baron Ashcombe), and a younger brother, Thomas Foulk Myddelton.
[4] His mother was the youngest daughter of William Nevill, 1st Marquess of Abergavenny.
In 1942 he took the Staff College, Camberley War Course, and subsequently commanded the 1st (Armoured) Battalion Coldstream Guards 1942–1944, including during the Normandy landings after D-Day and the advance across north west Europe.
On 27 July 1931, Myddelton was married to (Mary) Margaret Elizabeth Mercer Nairne (1910–2003), daughter of Major Lord Charles Fitzmaurice and his wife, the former Lady Violet Elliot-Murray-Kynymound (daughter of Gilbert Elliot-Murray-Kynynmound, 4th Earl of Minto).
[8] Together, Lady Margaret and Ririd were the parents of:[9] Myddelton died on 7 February 1988.