Wheeler moved to become Director of the
London Museum in 1926, and Cyril Fox replaced him as Director of the National Museum of Wales. Grimes became an assistant keeper of archaeology at the National Museum of Wales, working with the newly appointed keeper of archaeology,
Victor Erle Nash-Williams. He received an MA from the University of Wales in 1930 for a dissertation on the
Roman pottery from the
20th Legion's works at
Holt (then in
Denbighshire). He became interested in the prehistory of Wales, and was involved in excavations at
Pyle, Ludchurch, Corston Beacon and
Llanboidy. Grimes became a Fellow of the
Society of Antiquaries of London in 1934. He published a book on the prehistory of Wales in 1939,
Guide to the Collection Illustrating the Prehistory of Wales, which won the Cambrian Archaeological Association's
G. T. Clark prize in 1949. The book was republished as
The Prehistory of Wales in 1951, and a second edition followed in 1959. He met
Audrey Williams in 1935, while preparing an exhibition in Swansea for the centenary of the
Royal Institution of South Wales. They went on to work together at many excavations in Wales and elsewhere, and were later married. Grimes moved to Southampton in 1938 to become an assistant archaeology officer with the
Ordnance Survey, and was quickly involved in the excavation of the newly discovered ship burial at
Sutton Hoo the following year. He was seconded to the
Ministry of Works in the Second World War, and worked with Audrey Williams on quick surveys and excavations before the construction of new airfields and other military structures. His discoveries included an Iron Age religious site at
Heathrow. In 1945, he succeeded Mortimer Wheeler as director of the London Museum, then based in
Lancaster House. He was involved in the programme to excavate
Blitz sites in London before they were redeveloped. For this work, he received the freedom of the
City of London in 1952. A highlight was the excavation of the
London Mithraeum with Audrey Williams, which was discovered at a building site at
Walbrook in 1954. The site was featured in the
Illustrated London News illustrated with drawings by
Alan Sorrell. The unexpected discovery of a bust of
Mithras on the last scheduled day of the excavation generated considerable press and public interest, debates in Parliament and discussion in the Cabinet. The excavation was extended, allowing further discoveries to be made, but delaying the construction. Although Bucklersbury House was built over the site, Grimes succeeded in salvaging many of its finds and features including marble statuary attesting to the wealth of its congregation. The temple was reconstructed nearby in the 1960s, but the work was not supervised by archaeologists and Grimes was dismissive of the result. Grimes was appointed
CBE in 1955. He continued his excavations in London after he succeeded
V. Gordon Childe as director of the
Institute of Archaeology and professor of archaeology at the University of London in 1956 (Wheeler had founded the Institute in 1937, and Childe became director after Wheeler resigned in 1946). While Grimes was its director, the Institute moved from
St John's Lodge in
Regent's Park to new premises at
Gordon Square. Grimes remained interested in the archaeology of Wales. He received an honorary DLitt from the University of Wales in 1961, and was president of the
Cambrian Archaeological Association in 1963–64. He served on many commissions and committees with a variety of official bodies and archaeological societies and organisations, including the
Society of Antiquaries, the
Royal Archaeological Institute, and the
Council for British Archaeology. He was a member of the
Royal Commission on the Ancient and Historical Monuments of Wales for 30 years from 1948, also serving a period as chairman, and joined the
Royal Commission on the Historical Monuments of England in 1964. He retired from both commissions in 1978. Between 1975 and 1988 he served as the first chairman of the Dyfed Archaeological Trust. ==Private life==