Knubel became an official mountain guide in 1904 upon receiving his guide's licence, while continuing to work as a stonemason and tree feller. His mountaineering skills were noticed by British climber Geoffrey Winthrop Young, who hired Knubel as his guide. From 1906 to 1914, Young and Knubel climbed together each summer, making a number of first ascents together, including the Täschhorn's south face, the Weisshorn's southeast and northeast faces, the Rimpfischhorn and Zinalrothorn's east faces, the Gspaltenhorn's west face, Grandes Jorasses' west ridge, and Mont Brouillard. The route the two mapped out to reach the summit of Aiguille du Plan in 1907 is still the route used by most alpine climbers today. On Young's first ascent of the east face of the Aiguille du Grépon with several other climbers and Knubel and Henri Brocherel as guides in 1911, Knubel used an ice axe instead of a piton to clear an overhang; the spot was thereafter known as The Knubel Crack.