Bresci was born in Tuscany. He emigrated from Italy to the U.S., making his living as a weaver in Paterson, New Jersey, which had a large Italian-American anarchist community. He was one of the founders of La Questione Sociale, the Italian-language anarchist paper published in Paterson. According to Emma Goldman, "he was a skillful weaver, considered by his employers a sober, hard-working man, but his pay averaged only fifteen dollars a week. He had a wife and child to support; yet he managed to donate weekly contributions to the paper. He had even saved a hundred & fifty dollars, which he lent to the group at a critical period of La Questione Sociale. His free evenings and Sundays he used to spend in helping with the office work and in propaganda. He was beloved and respected for his devotion by all the members of his group".
In 1898, high bread prices led to demonstrations all over Italy. In Milan, an unarmed crowed of protestors marched toward the palace, which was surrounded by a strong military force under command of General Bava Beccaris. The people ignored the order to disperse, whereupon the General gave the signal that resulted in a massacre of hundreds of the demonstrators. King Umberto later decorated Beccaris, complimenting him upon his "brave defense of the royal house" -- as a result of which Bresci determined to kill the king. Bresci had his load to the paper returned (without telling his comerades why), and with the money he went to Italy. In Monza, where the King was visiting on July 29, 1900, he shot him three times.
Bresci was captured and put on trial, where he was defended by the anarchist lawyer Francesco Saverio Merlino. He was sentenced in Milan on August 29, 1900, to hard labor at Santo Stefano prison on Ventotene Island, where numerous other anarchists had also been sent over the years. There he was found dead in prison less than a year later, which was attbuted to suicide, although he was almost certainly murdered by the guards.