Green did not play high school basketball until his sophomore year, when he played junior varsity for J.Frank Dobie High School. In his junior year he made the varsity team, but his play was cut short because of academic issues.
He 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 was the game's 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 US. Green also won the McDonald's All-American Slam Dunk Contest that year, defeating future Duke player Josh McRoberts.
Green was one of the last players to enter the NBA directly from high school, as the collective bargaining agreement between NBA owners and the NBA Players' Association now mandates that American players who enter the NBA Draft must be at least one year removed from the graduation of their high school class and reach age 19 no later than December 31 of the calendar year of the draft.
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.
He appeared in 32 games and averaged 11.8 minutes, 5.2 points, and 1.2 rebounds during these contests. Gerald Green left marks on the league with his highlight plays including a shot clock beating half court three against Indiana and two Sports Center top play dunks in the closing seconds in two games against Toronto.
The season was going very well for Green, on April 10, 2007, Green scored a career high 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 second round draft pick, and cash considerations. He was subsequently released by the Rockets on March 8th and became an unrestricted free agent in July.
On July 1 2008, Gerald 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 was not supposed to go to the summer league but chose to go to learn the plays and get a feeling of the Mavs organization early and not let his chance slip away. In the Vegas Summer League he averaged an underwhelming 13.3 ppg and 3.8 rpg on 38% shooting from the the field. On a bright note, he was 43% from beyond the arc. Following the Vegas Summer League, the Mavs and Gerald participated in the Rocky Mountain Revue, a summer league held in Salt Lake City, Utah. There in just 25.8 mpg Green averaged 17.7 ppg to go with 2.7 rpg, 1.8 apg, 1 spg, .5 bpg, and a respectable 2.2 topg. This time around, Gerald shot a much more impressive 45%, including going 12 for 26 from behind the arc (46%), and 28-31 from the free throw line (90%). His overall performance was enough to earn him All-Revue Team honors.