Nash-Williams spent his entire professional career in the service of the National Museum of Wales and of the University College, Cardiff. When Mortimer Wheeler whose early pupil he was, became director of the National Museum in 1924, Nash-Williams was offered the Assistant Keepership of the Department of Archaeology under Cyril Fox (see above); and when Fox in his turn became Director, Nash-Williams became Keeper of the department and also lecturer in archaeology at the College, a post conjoint with the keepership since Wheeler's arrival in 1920; Nash-Williams, however, was destined to be the last holder of both positions. His interests lay mainly in the Roman and Early Christian periods. He was an enthusiastic and indefatigable excavator, having imbibed Wheeler's early teaching, but developing no further his best excavation-reports, thus, are his earliest — on Jenkins's Field and the Prysg Field, Caerleon, and on the baths, etc., and the defences of Caerwent. In connexion with Caerwent, he excavated at the Late Iron Age hillfort at Llanmelin and at Sudbrook promontory fort on the Severn coast; later, he re-excavated the Roman villa at Llantwit Major, and at the time of his death was engaged in an important series of ‘digs’ outside the legionary fortress of Caerleon. His earliest general publication in the Roman field was his Catalogue of the Roman inscribed and sculptured stones found at Caerleon (Nat. Mus. of Wales, 1935), in which his brother, Alva Harry, was co-author; his last such, The Roman frontier in Wales (Cardiff, 1954), ‘severely factual’ (H.J. Randall see below). In both these, he showed a decided ability to analyse, condense, and arrange diverse materials; and this character was exemplified to a greater and even more valuable extent in the one work of his which may justly be claimed to be a masterpiece, and into which the love and labour of many years, and the spiritual feeling of a devout Christian, also went: his Early Christian monuments of Wales (Cardiff, 1950). The memorial inscription on bronze at the Legionary Museum of Caerleon, now facing the great imperial inscription of AD 100 which he himself discovered, ends with these lines:

He was gracious in life, exact in scholarship, fearless in advocating what he believed to be the truth, unfailing in friendship, and selflessly helpful to his colleagues, his staff, and his students.