League Baseball (MLB) debut on September 13, 2005, as a pinch hitter against the Cincinnati Reds, and gained his first hit the following day against Brian Shackelford. In 2006, he split time between the Iowa Cubs and the MLB team, hitting .328 in 53 games with the Chicago team. In 2007, Theriot made the opening-day roster as a utility player, but the early-season struggles of César Izturis lead to Theriot getting more starts and quickly became the starter. Izturis was traded in July. Also in 2007, Theriot was reunited with his former collegiate teammate Mike Fontenot. Theriot and Fontenot made up the double-play combination for the 2000 National Champion LSU Tigers. Theriot was notably versatile throughout the 2007 season, playing multiple positions defensively and hitting all over the batting order. Despite impressive numbers from Theriot in the lead-off spot, albeit a small sample size, the return of Alfonso Soriano resulted in Theriot returning to second in the batting order. In 2008, Theriot had the sixth-best batting average in the National League. While not expected to hit for power, Theriot was asked to try to drive in more runs early in 2009 as the Cubs dealt with injuries and poor performance from their power hitters, including Aramis Ramírez, Alfonso Soriano, Geovany Soto, and Milton Bradley. He quickly went on a relative tear and smacked seven home runs, including his first two ever outside Wrigley Field, as the other players regained their health. Theriot hit his first career grand slam on May 1, 2009, against the Florida Marlins. The Cubs were down 5–2 when he hit it, and it gave them the lead. The Cubs won 8–6. On February 19, 2010, the Cubs went to salary arbitration with Theriot, the first time they had done so since 1993. Theriot's figure was one year, $3.4M. The Cubs' figure was also one year, but $2.6M. The Cubs won the case. He appeared in 96 games with the Cubs in 2010, hitting .284. Los Angeles Dodgers On July 31, 2010, Theriot and Ted Lilly were traded to the Los Angeles Dodgers for Blake DeWitt. He was assigned the number 13. He hit .242 in 54 games with the Dodgers. At second base, Theriot had a .996 fielding percentage. St. Louis Cardinals On November 30, 2010, Theriot was traded to the St. Louis Cardinals for Blake Hawksworth. Theriot wore number 3 on his uniform, as his