Posey, the oldest of four children, was born to Demp and Tracy Posey on March 27, 1987, in Leesburg, Georgia. His nickname, “Buster”, came from his father’s childhood nickname. He grew up a fan of the Atlanta Braves. He played football, soccer, and basketball growing up, but baseball was his main sport.

In Game 5 of the NLDS against the Reds on October 11, Posey hit a grand slam off of Reds starter Mat Latos to give the Giants a 6–0 lead and subsequently won the game. The Giants become the second NL team to win a Division Series after being down two games to none and first since the LDS became a permanent standard in the playoffs in 1995. Posey became just the third catcher in MLB history to hit a grand slam in the playoffs, along with Berra and Eddie Pérez. He also completed a strikeout-throw out double play at third base in the sixth inning to help preserve the victory. He batted .154 with four hits and one RBI in the NLCS as the Giants defeated the Cardinals in seven games. He hit a two-run home run against Max Scherzer in Game 4 of the World Series as the Giants swept the Detroit Tigers, giving Posey his second World Series ring.

During the college offseason, Posey started at shortstop for the Yarmouth-Dennis Red Sox in 2006 when they won the Cape Cod Baseball League championship. He started at catcher in 2007 when they won another championship.

In 2008, as a junior, he hit .463 with 26 home runs and 93 RBI, won the Johnny Bench Award, and garnered the Collegiate Baseball Player of the Year award. On May 12, he hit a grand slam and played all nine fielding positions in a 10–0 victory over Savannah State University; as a pitcher that day, he struck out both batters he faced. Posey was awarded the Dick Howser Trophy and the Golden Spikes Award at the end of the year.

Buster was considered by Baseball America to be the best catcher available in the 2008 MLB draft. He was drafted by the San Francisco Giants with the fifth overall pick. On August 16, the Giants signed Posey shortly before the signing deadline for draftees and gave him a $6.2 million signing bonus, the largest up-front bonus in Giants history. Entering the 2009 season, Baseball America ranked him the number two prospect in the Giants’ organization (behind Madison Bumgarner). He was invited to the Giants’ spring training in 2009. Following spring training, Posey was assigned to the Giants’ Class A Advanced affiliate, the San Jose Giants of the California League. In 80 games with San Jose, he batted .326 with 63 runs, 95 hits, 23 doubles, 13 home runs, and 58 RBI.

Because of an injury to Giants starting catcher Bengie Molina, Posey was called up to the MLB for the first time on September 2, 2009. On September 11, 2009, Posey made his MLB debut, striking out in his first at bat against Hiroki Kuroda of the Los Angeles Dodgers. Posey got his first major league hit on September 19 against Jeff Weaver of the Dodgers. In 17 at-bats with the Giants in 2009, Posey had two hits.

Posey married his high school sweetheart, Kristen, on January 10, 2009. They had twins (a son, Lee, and a daughter, Addison) in 2011. They adopted twin girls Ada and Livvi in 2020. Posey lives with his family in the East Bay during the season, and they also have a home in Georgia during the off-season.

Coming into 2010, Baseball America ranked Posey as the top prospect in the Giants’ organization. After again appearing in the Giants’ spring training camp, Posey began the 2010 season at Fresno, batting .349 with 31 runs scored, 60 hits, 13 doubles, six home runs, and 32 RBI in 47 games.

Posey was called up to the major leagues on May 29, 2010, and started at first base against the Arizona Diamondbacks. Posey drove in the first runs of his major league career, going three for four with three RBIs. He appeared primarily at first base through the end of June. Posey hit his first career home run against Aaron Harang of the Cincinnati Reds on June 9. Following Molina’s trade to the Texas Rangers on June 30, Posey became the starting catcher for the Giants.

Eligible for salary arbitration for the first time, Posey signed a one-year, $8 million contract with the Giants prior to the 2013 spring training season. On March 29, Posey agreed to an eight-year contract extension worth $167 million, said by the Giants to be the most lucrative in franchise history. The contract wiped out three arbitration years and five years of free agency for Posey, locking in his services through the 2021 season with a club option for 2022. The agreement was the second largest in major league history for a catcher, exceeded only by that of Joe Mauer in 2010 with the Minnesota Twins.

Posey set season highs with three hits and four RBI on April 6, 2011, including a two-run home run against Tim Stauffer in an 8–4 victory over the Padres.

Posey’s younger sister, Samantha, played softball for Valdosta State University. On April 18, 2011, she hit a home run cycle in a doubleheader.

Posey started at catcher during Matt Cain’s perfect game on June 13, 2012, the 22nd in major league history. He stated afterwards that the game had him feeling “as nervous as I’ve ever been on a baseball field.” He played in the 2012 Major League Baseball All-Star Game, his first, on July 10, going 0–2 with a walk and a run scored. After batting .289 with 10 home runs and 43 RBI in 77 games before the All-Star break, Posey batted .385 with 14 home runs and 60 RBI in the final 71 games of the season. On July 17, he had three hits and five RBI in a 9–0 victory over the Braves. Four days later, he had four hits and three RBI, including a two-run home run against Cole Hamels in a 10-inning, 6–5 victory over the Phillies. Two days later, he had three hits and four RBI, including a three-run home run against Clayton Richard in a 7–1 victory over the Padres. He hit a game-winning three-run home run against Lance Lynn on August 7 in a 4–2 victory over the St. Louis Cardinals. On September 17, he had three hits, including a game-winning two-run home run against Wade Miley in a 3–2 victory over the Diamondbacks. In 148 games, Posey had 78 runs scored, 178 hits (tied for eighth in the NL with David Wright), 39 doubles (tied for eighth with Yonder Alonso), 24 home runs, and 103 RBI (sixth).

Posey’s teammate Melky Cabrera batted .346 in 2012, but MLB declared him ineligible for the batting title after receiving a 50-game suspension for raised testosterone levels. As a result, Posey led both leagues in batting in 2012 with an average of .336, becoming the first catcher to lead the NL in hitting since Ernie Lombardi of the Boston Braves in 1942. He also became only the second San Francisco Giant to win the batting title, following Barry Bonds in 2002 and 2004. Posey’s .433 batting average against left-handed pitching (71 for 164) led all batters in the major leagues in 2012.

On July 1, 2013, Posey was named National League Player of the Week for the week of June 23 – 29. It was the second time Posey earned the award during his career, and the first since 2010. In six games Posey hit .500, to raise his average from .307 to .322, had an on-base percentage of .560, a slugging percentage of 1.182, stroked four home runs, and drove in six.

On July 13, 2013, Posey caught Tim Lincecum’s first no-hitter against the San Diego Padres. The no-hitter also marks the 15th ever in Giants history as well as the second performed in an away ballpark.

On July 16, 2013, Posey played in his second straight All-Star Game, striking out in his only at-bat.

Posey’s offense regressed in 2013 following his 2012 MVP season, especially in the second half of the season where he hit just 3 home runs. However, he still finished with a solid .294 batting average, 15 home runs and 72 runs batted in.

On July 13, 2014, Posey and pitcher Madison Bumgarner hit grand slams against the Arizona Diamondbacks. It marked the first time in Major League Baseball history that a catcher and a pitcher hit grand slams in the same game.

On August 29, 2014, in a 13–2 win over the Milwaukee Brewers at Oracle Park (then known as AT&T Park), Posey went 5-for-5 in 6 innings, and became the only catcher in Giants franchise history to have two career five-hit games.

Posey finished the 2014 season with a .311 batting average, 22 home runs, and 89 RBIs. In the 2014 World Series, the Giants defeated the Kansas City Royals in seven games, giving Posey his third championship in five years.

Posey caught Santiago Casilla’s immaculate inning save in May 2015 against the Cincinnati Reds, the first time that the Giants had struck out all three opposing batters in an inning on nine pitches since Trevor Wilson accomplished the feat in 1992.

On June 9, 2015, Posey caught rookie Chris Heston’s no-hitter against the New York Mets at Citi Field, including the final out, a strikeout. It marks the seventeenth no-hitter in Giants franchise history. This was the third no-hitter during which Posey played catcher and the fourth overall, as he started at first base in Tim Lincecum’s second career no-hitter. This puts him one behind the record for no-hitters caught, held by the Philadelphia Phillies’ Carlos Ruiz and the Boston Red Sox’ Jason Varitek.

On June 19, 2015, in a 9–5 win over the Los Angeles Dodgers at Dodger Stadium, Posey hit a grand slam and stole a base, becoming the first Giants catcher in 112 years since Roger Bresnahan to accomplish the feat. Four days later, in a 6–0 win over the San Diego Padres at Oracle Park, Posey hit another grand slam. On July 5, 2015, Posey was selected to his third career All-Star Game and was number one in voting for NL catchers. On September 6, 2015, at Coors Field, in a 7–4 win over the Colorado Rockies, Posey hit his 100th career home run.

On November 11, 2015, Posey was named winner of the 2015 Wilson Defensive Player of the Year award, given to the best defensive catcher in MLB. The following day, Posey was named winner of the 2015 National League Silver Slugger award at catcher. Posey finished 2015 with a .318 batting average, 19 home runs, and 95 RBIs.

Posey was selected by Fan Voting to start at catcher in the 2016 MLB All Star Game, Posey’s fourth career MLB All-Star Game, the most all-time by a Giants catcher in franchise history. He was the catcher for battery mate Johnny Cueto, the second battery mate he started and caught for in the Midsummer Classic after teammate Matt Cain in 2012.

In 2019, he batted .257/.320/.368 with 43 runs, 7 home runs, and 38 RBIs in 405 at bats. His .302 career batting average was 8th-best among active major league players.