During his student years, Lethbridge had frequented the Cambridge University Museum of Archaeology and Ethnology, where he had befriended its curator, Louis Clarke. Upon Lethbridge's graduation, Clarke offered him the honorary position of Keeper of Anglo-Saxon Antiquities at the museum. Lethbridge took up the voluntary and unpaid post, being able to subsist on his family's finances. There, Lethbridge befriended the archaeologist Cyril Fox, aiding in Fox's excavation of the Devil's Dyke in Cambridgeshire. Lethbridge's archaeological excavations were however deemed of poor quality by many of his peers. In turn, Lethbridge derided the archaeological establishment, being frustrated by how long it took them to accept what he deemed to be "facts", and he believed in focusing on what he believed to be instinct and common sense. There were exceptions to his scorn; for instance, he developed a friendship with T. D. Kendrick, an Anglo-Saxon art historian who then worked as Keeper of the British Museum in London. Over the course of his career at the museum, Lethbridge produced 60 archaeological reports, written in an unusually informal manner that used humour and wit and included narrative descriptions of the excavation process.