Domínguez made his Champ Car debut the following year in 2002 and became Rookie of the Year, with his first Champ Car victory in 2002 at the Honda Indy 300, although this was in a rain-shortened race largely dictated by the safety car. He was more competitive in 2003, winning the Grand Prix Americas in Miami. In 2004 Domínguez had his best season to date by finishing 5th in the final standings. This success with Herdez Competition allowed him to sign with the Forsythe team for 2005. However, Domínguez saw limited success at Forsythe. After the first four races of the 2006 season where he crashed into his teammate Paul Tracy on the first lap twice, Domínguez and Forsythe parted ways. Before the next Champ Car race in Portland, Domínguez signed on to Dale Coyne Racing to drive the Sonny's BBQ Ford Lola-Cosworth. For the final 3 races of the season, Domínguez secured sponsorship from Mexican state-owned petroleum company Pemex to race for the slightly more competitive Rocketsports team. Along with Champ Car racing, Mario tested a Jordan Grand Prix Formula One racing car at the Silverstone circuit in Northamptonshire, England, in 2005.