He threw and batted left-handed and played in Major League Baseball (MLB) for the Kansas City Royals, Colorado Rockies, Cincinnati Reds and San Francisco Giants.

After one season with the Cincinnati Reds, he signed with the San Francisco Giants in 2009, where he was a member of the 2010, 2012, and 2014 World Series championship teams.

His father was a member of the United States Air Force, and Affeldt lived in Guam, Merced, California, and Spokane, Washington, growing up.

[4] Affeldt ultimately decided not to accept a scholarship offer to play college baseball for his hometown Gonzaga University.

[6] In 1999, Affeldt spent the entire season with the Royals' single-A affiliate, which had changed to the Charleston Alley Cats during the offseason.

While with Wilmington, he led the entire Kansas City Royals' organization (and the Carolina League) with fifteen losses (to go with only five wins).

After getting pulled in the fourth inning due to an injury,[15] Affeldt was placed on the disabled list with fingernail and blister problems.

[7][16] After making three rehab starts in Wichita, Affeldt returned to the Royals on August 2 and threw a perfect inning against the Minnesota Twins.

[17] In 2003, Affeldt found himself competing with Runelvys Hernández for the first spot in the Royals' starting rotation during Spring training.

However, he did not get a save opportunity until April 16 against Detroit, and he was unable to finish that game because of a groin injury that landed him on the disabled list again.

[7] Affeldt returned to Kansas City's starting rotation in 2006 when he was named the fourth starter during Spring training.

[49] Affeldt recalled, "When [Royals General Manager Dayton Moore] traded me, he said, 'I feel like I need to give you a fresh start.

Affeldt pitched in all four games of the World Series without giving up a run, but the Rockies were swept by the Red Sox.

Affeldt got Brian Giles to ground into a double play to end the inning, but he gave up two runs (one earned) in the seventh as the Padres won 6–1.

[7][62] On May 31, he entered a game against St. Louis with runners on first and third and Albert Pujols on deck in the eighth, and he struck out Joe Thurston to end the inning and help the Giants win 5–3.

[63] The streak did not end until July 28, when Brandon Medders allowed one of Affeldt's runners to score in a game against Pittsburgh.

[64] On August 2, with runners on first and second with one out in the eighth inning against the Philadelphia Phillies, he retired Chase Utley and Ryan Howard to help the Giants win 7–3.

[78] On September 8, 2011, Affeldt suffered a season-ending injury as he gashed his non-throwing (right) hand trying to separate frozen hamburger patties and underwent surgery to address nerve damage.

In Game 4 of the 2012 World Series, Affeldt provided key relief, striking out Detroit Tigers power hitters Miguel Cabrera, Prince Fielder, and Delmon Young in the eighth inning to preserve a 3–3 tie.

The Giants would go on to win the game in the tenth inning, completing a sweep of the Tigers, earning Affeldt his second World Series ring.

Affeldt was credited with the win when Travis Ishikawa hit a walk-off home run in the bottom of the ninth to clinch the pennant for the Giants.

[90] Affeldt earned the win in Game 7 of the 2014 World Series against his first Major League team the Kansas City Royals.

On getting the Game 7 win at Kauffman Stadium, where he started his career, Affeldt said, "To be able to do it in Kansas City, with my past history, it just means a lot to me.

[95] Affeldt returned July 22, but on August 24, he suffered another off-the-field injury, a left knee subluxation, while spending time at a lake with his family on an off-day.

The ceremony included speeches from president and chief executive officer Larry Baer, manager Bruce Bochy, and starting pitcher Matt Cain, before Affeldt emotionally addressed the crowd.

[99] He made his final Major League appearance out of the bullpen later that day, pitching 0+2⁄3 innings and retiring the only two hitters he faced.

Bochy made a pitching change, allowing Affeldt to receive a standing ovation from his family, friends, and Giants announcers, coaches, executives, fans, and teammates as he walked off the mound.

[110] In October 2010, during a visit to a Bay Area elementary school alongside former Giants pitcher Juan Marichal, Affeldt told the students, "I try to make sure everybody, all the young people in the world, also have a chance to dream really big.

[112] In 2013, Affeldt released a book entitled To Stir a Movement: Life, Justice, and Major League Baseball.

In the book, Affeldt recounted a contract situation in 2010 where he voluntarily returned $500,000 to the Giants after discovering a clerical error.