In 1961, Cassin was both leader of and a successful ascentionist on the expedition that made the first ascent of the eponymous "Cassin Ridge" on Mount McKinley in Alaska, at that date the most technical route on the mountain. After this ascent, President Kennedy sent Cassin a telegram of congratulations and, had the Bay of Pigs crisis not intervened, he was scheduled to meet Cassin. In 1975, Cassin led an expedition to the then-unclimbed south face of Lhotse in the Himalaya, but this attempt was unsuccessful as a result of bad weather.