Affeldt was a third-round draft pick by the Kansas City Royals in 1997 MLB draft. He made the team in 2002, and started part of the year for them. In five seasons with the Royals, Affeldt bounced back and forth between the starting rotation and the bullpen. In 2006, he was traded to the Colorado Rockies at the trade deadline, and was a member of the Rockies 2007 World Series team. 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.