Born in Negrar, Province of Verona, Tommasi started his professional career with local club Hellas Verona FC, in Serie B. He made his Serie A debut on 7 September 1996 with A.S. Roma in a 3–1 win over Piacenza Calcio, and would be an instrumental figure in the side's 2001 conquest of the "scudetto", with manager Fabio Capello even labelling him as the team's most important player.