Croz began his guiding career in 1859 when he was engaged by William Mathews for an ascent of Mont Blanc. As well as making the first ascent of some of the most significant unclimbed mountains in the Alps - the Grande Casse, Monte Viso, the Barre des Écrins and the Aiguille d'Argentière - he also made the first traverse of many previously uncrossed cols, including the col des Ecrins, the col du Sélé and the col du Glacier Blanc in the Massif des Écrins (all in 1862 with Francis Fox Tuckett, Peter Perren and Bartolomméo Peyrotte). In 1863, he climbed the Grandes Rousses with William Mathews, Thomas George Bonney and his brother Jean-Baptiste Croz, and in 1864 he made the first traverse of the brèche de la Meije and the first traverse of the col de la Pilatte (with Edward Whymper, Horace Walker, A. W. Moore and Saas-Fee guide Christian Almer). Of the latter expedition, Whymper was later to write,