Born in Somerset to a wealthy family, Lethbridge was educated at Trinity College, Cambridge, during the course of which he attended an expedition to Jan Mayen island, becoming part of the first group to successfully climb the Beerenberg. After a failed second expedition to the Arctic Circle, he became involved in archaeology. In his capacity as Keeper of Anglo-Saxon Antiquities at the Cambridge University Museum of Archaeology and Ethnology, Lethbridge carried out excavations at various sites around Britain. His claims regarding the existence of Iron Age hill figures on Wandlebury Hill in Cambridgeshire caused significant controversy within the archaeological community, with most archaeologists believing that Lethbridge had erroneously misidentified a natural feature. Lethbridge's methodology and theories were widely deemed unorthodox, and in turn he became increasingly critical of the archaeological profession.