Carter attended Southwest DeKalb High School in Georgia where he was a three-sport athlete (football, baseball and basketball) and was named the starting quarterback as a junior.
In his final year, he led the Panthers to the 1995 AAAA State Championship under Coach William Godfrey, making it the first all African American school to achieve this milestone at the highest competition level in the state of Georgia. He also received Parade All-American, All-state (second in a row) and Georgia Player of the Year honors. His teammates included future Olympic athletes Angelo Taylor and Terrence Trammell.
Carter originally signed a football letter of intent with Georgia Tech in 1996, but opted instead to play minor league baseball after being selected by the Chicago Cubs as an outfielder 52nd overall in the 1996 MLB Draft.
With the Gulf Coast Cubs of the rookie league in 1997, Carter played in 55 games and hit .215 The following year, he was promoted to the Rockford Cubbies of Single-A and hit .211 in 105 games. He appeared in 28 games for Rockford in 1998, hitting .248 in 27 games. His final year in 1999, he went 0-for-3 in one game for the Daytona Cubs of Advanced A ball and left the team after only three games to return to Athens. In November 1999, he announced that he would resume his baseball career, but he never joined the Cubs in spring training.
Struggling with his baseball career, Carter opted to return to play college football in 1998, but made the highly publicized decision to sign with the University of Georgia instead of Georgia Tech, who officially contested the move but was still overruled by the NCAA.
After spending two years away from football, he won the job of starting quarterback in a highly contested battle (over future University of Oklahoma starter, Nate Hybl, among others), becoming the Bulldog's first freshman starter in 53 years. He also played a key role in the team finishing with a 9-3 record and winning the Peach Bowl. He registered 2,484 passing yards, which at the time was the fourth highest total and the second best freshman performance in school history. He was named Southeastern Conference Freshman of the Year and was recognized as one of the top young quarterbacks in the nation. He also showed he was a dual-threat quarterback, as he demonstrated in Georgia's 28-26 win over the Kentucky Wildcats on October 24, with the freshman running 14 times for 114 yards (including a 49-yard touchdown run) and completing 10-of-14 passes for 147 yards and 2 touchdown passes.
Carter had an impressive sophomore season, in which he posted 2,713 passing yards, 17 passing touchdowns, 5 rushing touchdowns, and 6 interceptions. Carter helped lead a young team to an 8–4 record and an Outback Bowl win.
As a junior, he displayed inconsistency, which included a 5 interception game against the University of South Carolina. He also suffered a torn ulnar collateral ligament in his right thumb during the eighth game of the season against the University of Florida and missed the rest of the year including the Oahu Bowl. He was replaced with junior Cory Phillips. He recorded 91 of 183 completions, 1,250 passing yards, 6 touchdowns and 10 interceptions.
Carter was 23-8 as the starting quarterback for the Georgia Bulldogs and declared for the NFL Draft after his junior season. He started all but one game in his college career, finishing second in school history in passing yards (6,447), career pass attempts (853), career completions (483), career offensive plays (1,104), career total offense (7,053 yards) and was ranked third in career touchdown passes (35).
In the 2001 NFL Draft, the Dallas Cowboys didn't have a first-round pick because of the trade that sent two first round choices to the Seattle Seahawks in exchange for wide receiver Joey Galloway. Looking for a replacement to the recently retired Troy Aikman, Carter was selected by the Dallas Cowboys in the second round (53rd overall). At the time, it was a selection that was criticized by the media as a reach and it was later reported that owner/general manager Jerry Jones influenced the organization into making it.
Although he was expected to spend time learning the game behind starter Tony Banks, after having a notable pre-season he was named the team's new starting quarterback, becoming the first rookie quarterback selected in the second round, to start a week 1 game in NFL history and only the third rookie quarterback to open the season as a starter in franchise history. However, he was also part of a succession of short-tenured quarterbacks following the retirement of Aikman. After suffering two separate injuries, he ended up starting only eight games, both Anthony Wright and former San Diego Chargers second-overall pick Ryan Leaf started three, while former Arkansas Razorback Clint Stoerner started two.
The highlight of his rookie season was a 27-21 win against the San Francisco 49ers (finished 12-4) in Week 16. Carter showed promise with 241 yards passing and two touchdowns, to become just the second Cowboys rookie to win NFC Offensive Player of the Week honors. He also had an important 20-13 victory over the New York Giants in which Carter threw for nearly 200 yards, scrambled for a first down late in the game, and threw the game-winning touchdown pass to wide receiver Antonio Bryant.
The next year the Cowboys would sign another young quarterback and former baseball player, Chad Hutchinson, to compete with Carter. He eventually lost the starting job to Hutchinson after a loss to the Arizona Cardinals in which he engaged in a heated sideline argument with Jerry Jones. The highlight of the 2002 season was Carter leading Dallas to a dramatic come from behind win for the second time in three weeks (the other one was against the St. Louis Rams), on his 25th birthday he turned a 13-0 deficit to the Carolina Panthers, into a 14-13 victory by throwing an 80-yard touchdown pass to Joey Galloway with 3:55 minutes left, then a 24-yarder to Antonio Bryant with 56 seconds to go.
In his third season, under newly hired head coach Bill Parcells, Carter retook the starting role and brought stability to the quarterback position. He led the Cowboys to a 10-6 record and a playoff appearance.
During the offseason, coming off a successful year, Carter was abruptly cut on August 4, 2004 under unclear circumstances. The group of quarterbacks for the Cowboys that offseason had expanded with the trade for yet another former baseball player, Drew Henson, and the acquisition of Vinny Testaverde off waivers. Before Carter's release, it had been projected that he had a slight edge over Testaverde for the starting role and that former third-string quarterback, Tony Romo, would be waived. League sources eventually revealed that Carter had been released after failing a drug test. He'd already flunked two previous tests and would have to be suspended for the first four games of the season.
In his Cowboys career, he started 31 games, registering 507 completions in 902 attempts for 5,839 yards with 29 touchdowns, 36 interceptions and a 72.3 passer rating.
Following his exit, Cowboys coach Bill Parcells shed some light into Carter's situation at Dallas: "I became pretty close with Quincy personally, and this kid had a lot of good qualities," Parcells said. "He was smart. He understood it. But I just couldn’t save his ass. I really couldn’t. "You just didn’t have the time. There he is, he got his team in the playoffs, he’s the starting quarterback of the Dallas Cowboys, he’s playing good, he’s improving, he can get out of trouble, he’s pretty smart, he can make almost every throw -- and it’s just, some people just can’t fight the pressure to succeed. They just can’t fight it. It’s too much on them once the bar gets up a little bit. It’s too much. I don’t know all the problems or the demons exactly, but that’s what eventually took him down".
On August 24, 2004, Carter was signed to a one-year contract by the New York Jets to serve as a veteran backup to Chad Pennington. He ended up starting three games (winning two) after Pennington injured his rotator cuff, and if not for his performance, the team would not have made the playoffs. He finished the season with some of the best statistics of his career: 35 completions in 58 passes for 498 yards, 3 touchdown, 1 interception and a 98.2 passer rating.
The Jets declared him inactive for the divisional playoff game against the Pittsburgh Steelers on January 15, announcing that he had left the team to attend to his sick mother, when in reality he had enrolled into a rehabilitation program. He was not re-signed during the 2005 offseason.
On April 4, 2006, Carter was signed by the Montreal Alouettes of the Canadian Football League to a one-year contract with an option for 2007 only to be cut by the team the following month. On the subject of being released, Carter remarked, "This is a joke... an insult." One CFL club official told the Montreal Gazette that Carter has "a serious marijuana problem."
In February 2007, Carter signed with the Bossier–Shreveport Battle Wings of the af2. Through the first three games of the 2007 season he was the third ranked passer, with a rating of 124.3, throwing for 18 touchdown passes, but he was suspended indefinitely from the team in late May for missing team meetings. Battle Wings coach Jon Norris named Carter the starting quarterback for their June 16 game against the Corpus Christi Sharks and passed for a franchise-record eight touchdowns in the Battle Wings' 81-35 win.
Carter was arrested on possession charges by the Shreveport police on October 12, 2007. Because the incident marked the second time he was arrested for the same crime, the charge was a felony. He was released on his 30th birthday, on a bond of $5,224, according to an official in the records department at the Caddo Correctional Facility.
On June 2, 2008 Carter signed with the Arena Football League's Kansas City Brigade. The Brigade, who had one victory at the time of the signing, were hoping Carter's strong arm could resurrect their season. Herman Edwards, who coached Carter while with the Jets, commented on Carter's personality calling him of "good character". Carter wore #3 and started the last three games of the season, before being signed to a two-year contract extension.
On July 31, Carter had a workout with the Miami Dolphins but was not signed. On October 20, 2008, he was released by the Brigade.
Carter signed a one-year contract with the Abilene Ruff Riders of the Indoor Football League in March 2009. On May 10, 2009, he was arrested by Abilene police for an outstanding warrant, related to a DWI arrest in south Texas and subsequent probation violation. Carter was arrested again on June 18, 2009, for failing to pay his bondsman after his May 10 arrest. Returning from injury, Carter no-showed for the July 4 game and was subsequently cut from the team.
On February 19, 2015, he was signed by the Corpus Christi Fury of the American Indoor Football league.
On December 15, 2006 Carter was arrested in Irving, Texas on possession of marijuana charges. He was released in lieu of a $500 bond paid by Dallas-Fort Worth area sports talk-show host and journalist, Randy Galloway.
Carter became an independent youth football coach in Georgia specializing in training for the quarterback position. His personal problems continued until July 2013, when he was arrested again and charged with family violence simple battery after authorities said he allegedly threw a child safety seat at his girlfriend. The charge was a misdemeanor. Carter's stellar performance in college at the University of Georgia caught the attention of several rap artists in Atlanta including Gucci Mane who referenced Carter in his single "Trap House". Carter has a son named Quincy Carter, Jr. who attends Southwest DeKalb High School.