Kielty spent each season between 2001 and 2007 playing in the Majors, most notably with the Minnesota Twins and Oakland Athletics. Breaking into professional baseball as an undrafted free agent with Minnesota in 1999, Kielty would reach the Major Leagues just two seasons later with the Twins in 2001, appearing in 37 games. He played in 112 games with the Twins the following season in 2002, and batted .291 with 12 home runs and 46 RBI. That effort saw Kielty finish an impressive fourth in American League Rookie of The Year voting. Midway through the 2003 season, Kielty was traded from Minnesota to the Toronto Blue Jays for outfielder Shannon Stewart.

After finishing the ’03 season with Toronto, Kielty was traded again in the offseason, to the Oakland Athletics for starting pitcher Ted Lilly, and would spend the next three-plus seasons in Oakland. He finished 2007 with the Boston Red Sox, his most recent MLB stint.

In 599 Major League Games played over seven seasons, Kielty has accumulated 53 home runs and 253 RBI, with a .254 average. Primarily a corner outfielder during his career, he does have Major and minor league starts in centerfield and at first base as well.

Twice Kielty was on the losing end of an American League Championship Series, in 2002 when Anaheim defeated Minnesota, and again in 2006 when Detroit defeated Oakland. However, Kielty was on the Red Sox World Series roster when they captured the 2007 title over Colorado. In his only World Series at-bat during Game 4 on October 28, Kielty slugged a pinch hit home run off reliever Brian Fuentes on the first pitch of the top of the eighth inning at Coors field, extending Boston’s lead to 4-1. It proved to be the game winning run, as the Red Sox held off the Rockies 4-3, winning the series 4-0.