Cassin started mountaineering in around 1930 together with a group known as the Ragni di Lecco (spiders of Lecco). In 1934, he made the first ascent of the Piccolissima of the Tre Cime di Lavaredo. In 1935, after having repeated Emilio Comici's route on the north-west face of the Civetta, he climbed the south-eastern ridge of the Trieste Tower and, with Vittorio Ratti, established a new route on the north face of Cima Ovest di Lavaredo. In 1937, Cassin made his first climb on the granite of the Western Alps. Over the course of three days, 14-16 July, he made the first ascent of the north-east face of Piz Badile in the Val Bregaglia (Bergell), Switzerland, accompanied by Ratti and Gino Esposito after they teamed up with M. Molteni and G. Valsecchi, the latter two of whom died of exhaustion and exposure on the descent. This route is known today as the Cassin Route or the Via Cassin. He confirmed his extraordinary mountaineering skills 50 years later by climbing this route again at the age of 78.