Hornish began driving for Marlboro Team Penske during the 2004 IndyCar Series season in its No. 6 Dallara IR03-Toyota Indy V8, winning the season opener at Homestead-Miami in his first race with the team, passing teammate Hélio Castroneves on the final lap. After qualifying in 11th place at the Indianapolis 500, he battled for the lead with Buddy Rice and Dan Wheldon and led for nine laps (the first laps Hornish led in his Indianapolis 500 career). On lap 105, entering the main straightaway out of the fourth turn, Hornish tried to pass Darren Manning and Greg Ray's lapped car. They collided, crashing into the pit lane; Hornish finished 26th, dropping to ninth place in the point standings. The rest of his season was sub-par, with two podium finishes. Hornish was seventh in the championship standings with 387 points, his worst finish since he placed 21st in 2000.