Tomasson began climbing with Michele Bettega, a mountain guide, in 1897. Together, they made the first ascents of Cima d'Alberghetto, Torre del Giubileo, Campanile della Regina Vittoria, Monte Lastei d'Agner, and Sasso delle Capre. In 1898 she made the first ascent of the northeast face of Monte Zebrù, which was considered at the time to be the most difficult ice wall to climb in the Tyrol, as well as the first ascent of Ortler and the second ascent of the west face of Laurinswand, which was considered to be the Dolomites' most difficult rock wall. She and Luigi Rizzi were the first climbers to summit the Dent di Mesdi via the south face in 1900. In July 1901 Tomasson, Bettega and Bartolo Zagonel made the first ascent of the south face of the Marmolada, which is considered her greatest climbing achievement. The route had been considered the longest and most difficult climb in the Alps for more than a decade, yet Tomasson's team made the ascent in just one day.