Calvinism is named for one man and was not the work of one man. Huldrych Zwingli was reforming Zurich while Calvin was a schoolboy, and the Reformed tradition already had a shape, plainer worship, a symbolic reading of the Supper, a stricter rule that admits into worship only what Scripture commands, before Calvin arrived in Geneva in 1536.
What Calvin supplied was system. The Institutes of the Christian Religion, first published in 1536 and rewritten until the final Latin edition of 1559, is the most orderly statement of Protestant theology produced in the century, and it is the reason his name attached to the whole current. He also supplied a working model of a city under church discipline, run through a consistory of pastors and elders with real power over the morals of citizens, which impressed visiting exiles enormously and has coloured the tradition's reputation ever since.
Two things commonly said about him are worth correcting. He was not the ruler of Geneva; he was a salaried preacher who was expelled once and spent years in conflict with the city council, and he did not obtain citizenship until late. And predestination is not the organising principle of the Institutes: it appears well into the work, as a pastoral consolation, and only became the tradition's identifying badge through the polemics of the generation after him.
The execution of Michael Servetus at Geneva in 1553, for denying the Trinity, is the episode the tradition is most often asked about. Calvin pressed the prosecution, sought a less painful method of execution, and defended the principle afterwards. The judgement was the city's; the responsibility was shared, and Reformed writers have not generally tried to argue it away.