Posey grew up in Leesburg, Georgia. He played four sports in high school, when playing baseball; he excelled at hitting and pitching. He attended Florida State University, where he began playing the catcher position. He won the Golden Spikes Award in 2008 and was selected by the Giants with the fifth overall pick in the 2008 Major League Baseball Draft. Posey made his major league debut on September 11, 2009. After beginning the 2010 season in the minor leagues, he was called back up to the major leagues in May. He played first base when he came up, but became the Giants regular catcher at the end of June.