He joined a Venetian regiment at Corfu, his stay being broken by a brief trip to Constantinople, ostensibly to deliver a letter from his former master the Cardinal.[21] He found his advancement too slow and his duty boring, and he managed to lose most of his pay playing faro. Casanova soon abandoned his military career and returned to Venice.
At the age of 21, he set out to become a professional gambler, but losing all the money remaining from the sale of his commission, he turned to his old benefactor Alvise Grimani for a job. Casanova thus began his third career, as a violinist in the San Samuele theater, "a menial journeyman of a sublime art in which, if he who excels is admired, the mediocrity is rightly despised. ... My profession was not a noble one, but I did not care. Calling everything prejudice, I soon acquired all the habits of my degraded fellow musicians."[22] He and some of his fellows, "often spent our nights roaming through different quarters of the city, thinking up the most scandalous practical jokes and putting them into execution ... we amused ourselves by untying the gondolas moored before private homes, which then drifted with the current". They also sent midwives and physicians on false calls.[23]
Good fortune came to the rescue when Casanova, unhappy with his lot as a musician, saved the life of a Venetian nobleman of the Bragadin family, who had a stroke while riding with Casanova in a gondola after a wedding ball. They immediately stopped to have the senator bled. Then, at the senator's palace, a physician bled the senator again and applied an ointment of mercury to the senator's chest (mercury was an all-purpose but toxic remedy of the time). The mercury raised his temperature and induced a massive fever, and Bragadin appeared to be choking on his own swollen windpipe. A priest was called as death seemed to be approaching. Casanova, however, took charge and taking responsibility for a change in treatment, under protest from the attending physician, ordered the removal of the ointment and the washing of the senator's chest with cool water. The senator recovered from his illness with rest and a sensible diet.[24] Because of his youth and his facile recitation of medical knowledge, the senator and his two bachelor friends thought Casanova wise beyond his years, and concluded that he must be in possession of occult knowledge. As they were cabalists themselves, the senator invited Casanova into his household and he became a lifelong patron.[25]