Posey worked his way through the Giants' farm system in his first two seasons of professional baseball, advancing to its AAA affiliate. He made his major league debut on September 11, 2009, and after playing a third of the 2010 season in the minors, was called up to the Giants on May 29, becoming the team's starting catcher on June 30. In July 2010 he earned both NL Player of the Month and NL Rookie of the Month honors. He was also named the 2010 National League Rookie of the Year.
Posey played college baseball for the Florida State Seminoles under coach Mike Martin. He began his freshman year at FSU playing shortstop, starting all 65 games for the Seminoles, and was named a Louisville Slugger Freshman All-American. He finished his freshman season with a .346 batting average, four home runs and 48 RBIs. As a sophomore, he moved from shortstop to catcher on the suggestion of assistant coach Mike Martin, Jr. He batted .382 with three home runs and 65 RBIs. After just one season of playing the position, Posey was a finalist for the Johnny Bench Award, awarded to the best catcher in college baseball. He would end up finishing behind Edward Easley of the Mississippi State Bulldogs.
In 2008, as a junior, he hit .463 with 26 home runs and 93 RBIs, won the Johnny Bench Award and was named the 2008 Collegiate Baseball Player of the Year. On May 12, 2008, he played all nine fielding positions in a game against Savannah State, including pitcher where he struck out both batters he faced. On June 14, 2008, Posey was awarded the Dick Howser Trophy, and on July 16, 2008, he won the Golden Spikes Award, which is given to the best player in amateur baseball.
During the college offseason, Posey started at shortstop on the 2006 Cape Cod Baseball League championship team for the Yarmouth-Dennis Red Sox and started as the catcher on the 2007 championship team as well.