Mount Slesse was first climbed on August 10, 1927, by Stan Henderson, Mills Winram, and Fred Parkes, via a route starting from Slesse Creek. The standard route today is the Southwest Route, which involves approximately of ascent, a good deal of scrambling and some sections of technical rock climbing up to class 5.6. However Slesse is most famous for the Northeast Buttress, first climbed on August 28, 1963 by Fred Beckey, Steve Marts, and Eric Bjornstad. It is a serious multi-day rock climb (Grade V) with technical difficulty of class 5.8 or 5.9. This climb is featured in Roper and Steck's Fifty Classic Climbs of North America. In July 1966 Hans Baer and Jack Bryan did the second ascent of this route, with one bivouac and a straightline exit at the top of the buttress.