Green transferred to a charter school, Gulf Shores Academy in Houston where he repeated his junior year. In his senior year at Gulf Shores, he averaged 33 points, 12 rebounds, 7 assists, and 3 blocked shots per game. He was named an All-American and was the high scorer with 24 points in the 2005 McDonald’s All-American Game, which features two teams composed of the best high school basketball players in the United States. Green also won the McDonald’s All-American Slam Dunk Contest that year, defeating future Duke player Josh McRoberts.

Considered a five-star recruit by Rivals.com, Green was listed as the No. 1 player in the nation in 2005.

During the pre-season, Green averaged 8 points and 1.3 rebounds per game for the Celtics in the 2005 Las Vegas Summer League.

After seeing limited playing time during the first part of the season, Green was placed on the Fayetteville Patriots of the NBA D-League by the Celtics in January 2006. He was recalled and activated to an NBA roster position on February 3. He was reassigned to the NBDL, to the Florida Flame, on February 16. He was recalled and activated again on February 21, and saw his first significant NBA game action on the following night, scoring 13 points and grabbing 9 rebounds in 23 minutes.

On February 17, 2007, Green won the 2007 NBA Slam Dunk Contest, beating out Nate Robinson, Tyrus Thomas, and Dwight Howard.

The season was going very well for Green. On March 16, 2007, he scored a career-high 25 points in a Celtics loss to the Dallas Mavericks, and on April 10, 2007, Green topped that with 33 points (12-22 FG, 4-5 3FG) at Atlanta.

On July 31, 2007 it was made official that Green was to be traded along with Al Jefferson, Ryan Gomes, Theo Ratliff, Sebastian Telfair, and draft picks, to the Minnesota Timberwolves for Kevin Garnett.

At the 2008 NBA Slam Dunk Contest, Green opened the competition with a dunk he called “The Birthday Cake” in which he blew out a birthday candle on a cupcake set on the back of the rim while dunking. He followed that up with an alley-oop two-handed windmill. His first dunk of the second round was an alley-oop from behind the basket which he put between his legs before dunking. His final dunk of the night was a between-the-legs one-handed slam that he performed after taking off his shoes. He ended up finishing second to Dwight Howard, after Howard received seventy-eight percent of the votes cast by fans through text message.

On February 21, 2008, Green was traded to his hometown team Houston Rockets for Kirk Snyder, a 2010 second-round draft pick, and cash considerations. He was subsequently released by the Rockets on March 8 and became an unrestricted free agent in July.

On July 1, 2008, Green signed with the Dallas Mavericks. His agent Colin Bryant told Fox 26 Sports, “We have a verbal agreement with Dallas on a one year contract, We talked to a number of teams and there is more money out there, but this is the best situation for Gerald at this point in his career.”

Green signed with PBC Lokomotiv Kuban of Russia on December 2009. The Lokomotiv Kuban waived him on June 2010. He then was named to the Los Angeles Lakers roster for the 2010 NBA Summer League. After the Summer League, Green signed with BC Krasnye Krylya, also a Russian team. In October 2011 he moved to China, signing a one-year contract with the Foshan Dralions. On December 2, 2011, Green was released by Foshan.

On December 13, 2011, Green signed with the Los Angeles Lakers as an opportunity to return to the NBA. However, Green was waived by the Lakers on December 22 just days before the shortened regular season began.

On December 28, 2011, Green signed with the Los Angeles D-Fenders of the NBA Development League. He played at the NBA D-League All-Star Game and was awarded the NBA D-League All-Star Game Most Valuable Player. Over the season, he averaged 19.1 points, 2.6 rebounds and 1.6 assists per game. He recorded a season-high of 35 points against the Erie BayHawks.

The New Jersey Nets signed Green to a 10-day contract on February 27, 2012. He was signed to a second 10-day contract on March 8, 2012. His dunk against the Houston Rockets on March 11, 2012 was called “one of the dirtiest in-game windmill alley-oops in NBA history”. On March 18, 2012, he was signed for the rest of the season. Through the season he averaged 12.9 points, 1.1 assists and 3.5 rebounds in 31 games played.

On July 12, 2012, Green signed with the Indiana Pacers. During the 2013 All-Star weekend, Green participated in his third NBA Slam Dunk Contest.

On July 27, 2013, Green was traded to the Phoenix Suns alongside Miles Plumlee and a future first-round pick for Luis Scola. Green started for the Suns when fellow guards Goran Dragić and Eric Bledsoe were injured during their respective periods in November 2013. He received more starting time with the Suns after Bledsoe had a major knee injury during January, February, and early March 2014. Throughout this period, Green would take over some games as being the team’s leading scorer, including a 112–107 overtime victory against the Denver Nuggets, in which he’d record a then-career-high 36 points against them, which included making 8 of the team’s 13 points in overtime.

On March 6, 2014, Green recorded a new career-high 41 points, including a career-high 8 made three-pointers, in a 128–122 victory over the Oklahoma City Thunder. When Eric Bledsoe returned to action on March 12, 2014, Green returned to his bench role. Green finished the season with new career highs in points per game and free-throw percentage. As a result, he finished fourth in the NBA Most Improved Player Award behind Anthony Davis, Lance Stephenson, and Green’s teammate Goran Dragić.

On April 8, 2015, Green scored a season-high 30 points on 10-of-19 shooting in a 104–107 loss to the Dallas Mavericks.

On July 9, 2015, Green signed with the Miami Heat. He made his debut for the Heat in the team’s season opener against the Charlotte Hornets on October 28, scoring 19 points off the bench in a 104–94 win. On November 27, he scored a season-high 25 points while starting in place of the injured Luol Deng, helping the Heat defeat the New York Knicks 97–78. He topped that mark on April 1, 2016, scoring 30 points while starting in place of the injured Dwyane Wade, helping the Heat defeat the Sacramento Kings 112–106.

On July 27, 2016, Green signed with the Boston Celtics, returning to the franchise for a second stint. On December 27, 2016, he scored a season-high 19 points in a 113–103 win over the Memphis Grizzlies. On April 23, 2017, in Game 4 of the Celtics’ first-round playoff series against the Chicago Bulls, Green had a postseason career-high 18 points, 16 of which were tallied before halftime.

On September 24, 2017, Green signed with the Milwaukee Bucks. He was waived on October 14, after appearing in four preseason games.

On December 28, 2017, Green returned with the Houston Rockets for a second stint. On January 3, 2018, he scored a season-high 27 points in a 116–98 win over the Orlando Magic. Two days later, Green came off the bench to score 29 and tie a career high with eight 3-pointers in a 124–114 loss to the Golden State Warriors. He became the first player in franchise history to make seven or more 3-pointers in consecutive games after he made seven against Orlando. On March 30, Green hit a buzzer-beating three-pointer to snatch a win against the Phoenix Suns.

On July 10, 2018, Green re-signed with the Houston Rockets.

On July 22, 2019, Green re-signed with the Houston Rockets. On October 28, the Rockets announced that Green had undergone surgery to repair a broken bone in his left foot and would be sidelined for approximately six months.

On February 5, 2020, the Rockets traded Green to the Denver Nuggets in a four-team trade. The Nuggets later announced via Twitter that they had waived Green.