Loretta made his Major League debut on September 4, 1995 for the Brewers against the Minnesota Twins and recorded his first hit on September 10 against the Texas Rangers. He remained on the Brewers every day roster as a utility player through 2002. On June 20, 2001, during an 11–3 loss to the Cincinnati Reds, Loretta took the mound to pitch a scoreless eighth inning. Brewers managerDavey Lopes asked the team for a volunteer to pitch an inning in hopes of saving the depleted bullpen, and Loretta volunteered. He had not pitched since college. Loretta faced five batters, gave up one hit and one walk and got two strikeouts on 19 pitches. He was the first everyday player to pitch for the Brewers since 1991. Loretta was traded to the Houston Astros during the 2002 season for Keith Ginter and Wayne Franklin, and played in 21 games as a reserve before filing for free agency.
San Diego Padres
He signed with the San Diego Padres in 2003. His most productive season came in for the Padres, when he batted.335 with 47 doubles, 16 home runs, 76 RBI, 108 runs scored, and 208 hits – all career bests, and good enough to earn him a spot on the National League All-Star team. His.335 average ranked him third in the National League batting race behind Barry Bonds and Todd Helton, and he also joined Tony Gwynn as the only San Diego players to have 200 hits in a regular season. He was voted Padres team MVP in 2003 and 2004. In 2005, Loretta hit.280 with three home runs and 38 RBI in 105 games with the Padres, after he had surgery to repair a strained ligament in his left thumb.
On January 4, 2007, Loretta signed with the Houston Astros on a one-year contract to serve a utility role. On June 29, 2007, with 2 outs and behind one run, the Colorado Rockiesintentionally walkedCarlos Lee who had hit a walk-off grand slam the night before. Loretta then hit a 2-run homer for another walk-off home run the second night in a row.
Los Angeles Dodgers
On December 10, 2008, Loretta signed a one-year contract with the Los Angeles Dodgers for reported $1.25 million. On October 8, 2009, Mark Loretta hit a walk-off single to defeat the St. Louis Cardinals, 3–2, in game two of the National League Division Series. The score was tied and the bases were loaded with two outs when he stroked a line drive off Cardinal reliever Ryan Franklin to center fielder Colby Rasmus. Until this game-winning single Mark Loretta had been hitless in his career against Franklin. His single completed a come from behind victory for the Los Angeles Dodgers who trailed the Cardinals, 2–1, going into the bottom of the ninth inning. Loretta said, "That's the biggest hit of my career." This sudden victory enabled the Dodgers to take a two games to none lead in the division series. They would go on to win the series, 3–0.
Retirement
Loretta retired after the 2009 season. He joined the Padres as a special assistant to the baseball operations staff in 2010.
Loretta attended Saint Francis High School in La Cañada, California, and he was a classmate of Gregg Zaun. Loretta has a wife named Hilary, son named Frankie, and daughter named Lucy. His parents are David and Ellen Loretta, and he has a brother Chris and a sister Kelly. Loretta is also a member of the Fraternity of Phi Gamma Delta, commonly known as FIJI.