During his studies Lethbridge decided to join an expedition to visit Jan Mayen island in the Arctic Circle, alongside fellow Cambridge students James Chaworth-Musters and William Syer Bristowe and a don from St. John's College, Cambridge, the geologist James Wordie. Although initially planning to travel independently, they decided to join a Norwegian expedition led by Hagbord Ekerold and accompanied by Swiss mountaineer and glaciologist Paul Louis Mercanton. The expedition set sail from Bratvaag aboard two ships in August 1921, and upon arriving at Jan Mayen they became the first team to successfully climb the Beerenberg ('Bears Mountain'). During the expedition, Lethbridge was also able to explore his growing interest in archaeology by excavating at an abandoned Eskimo settlement. "The Times" recognised the expedition as the mountaineering event of the year.