In 1925, after graduating from Woolwich, Williams-Wynn was commissioned into the Royal Regiment of Artillery; He served in the Royal Horse Artillery, was an Instructor at the Equitation School, Weedon, Adjutant of the 61st (Carnarvon and Denbigh Yeomanry) Medium Regiment of the Royal Artillery (Territorial Army), from 1936 to 1940, and was promoted Major in 1940, having been appointed a Justice of the Peace in 1937. He was second in command of his regiment while it was part of the British Expeditionary Force to France and was at the evacuation of Dunkirk, then was posted to the Far East. During service with the 18th Infantry Division at Singapore he was twice mentioned in despatches, and after the fall of Singapore in February 1942 was a prisoner of war of the Imperial Japanese Army on the Burma Railway until the end of the war in 1945.