He served as the offensive coordinator of the North Texas Mean Green (2016–2018), USC Trojans (2019–2021) and the West Virginia Mountaineers (2022).

After high school graduation, he enrolled at Texas Tech University and played quarterback for the Red Raiders, starting for three seasons.

[8] The following season, he served as the backup quarterback behind another Texas Tech record setter, Cody Hodges, who led college football in passing yards in 2005.

Despite the 4–1 start, Harrell was benched halfway through the next game due to poor play after the Raiders fell behind the Missouri Tigers 24–0, and Todd was inserted for two series.

The Red Raiders, sparked by a 43-yard touchdown from Harrell to Joel Filani, went on to outscore the Gophers 31–0 through the end of regulation sending the game into overtime.

On the final drive, Harrell drove the Red Raider offense almost 70 yards in less than a minute to set up the game-tying field goal by Alex Trlica.

Following a Minnesota field goal, Tech scored a touchdown to win the game 44–41 capping a 31-point deficit, setting the record for the largest comeback victory in FBS bowl history.

The Raiders then dropped two games to Missouri and Colorado which knocked Tech out of the national conversation, but rebounded to beat Baylor 38–7.

The Raiders upset the # 4 ranked Oklahoma Sooners in Lubbock to finish the regular season 8–4 overall and 4–4 and third place in the Big 12 South.

[14] Harrell, along with coach Mike Leach and teammate Michael Crabtree were featured on the cover of the 2008 edition of Dave Campbell's Texas Football.

[16][17] After leading the Red Raiders to a victory over the #1 Texas Longhorns, Harrell and teammate Daniel Charbonnet were selected for Big 12 Player of the Week honors.

[20][21][22] Following the inaugural game of the 2008 season, where Texas Tech defeated the Eastern Washington Eagles, 49–24, Harrell was selected as the AT&T ESPN All-America Player.

[24][25] During the game against Kansas State, Harrell broke the all-time record for passing yards at Texas Tech.

On the day following the Baylor game, Harrell underwent a four-hour surgery, in which 17 pins and two plates were inserted into his injured hand to heal nine different breaks.

[32] On January 2, 2009, Harrell played his final collegiate game, a 34–47 loss to Ole Miss in the 2009 AT&T Cotton Bowl Classic.

In the game, Harrell finished his college career with an NCAA all-time record of 134 passing touchdowns, breaking the previous mark of 131 set by Colt Brennan.

[46] He participated in rookie camp for the Cleveland Browns in May 2009 but was not signed despite coach Eric Mangini giving Harrell praise.

[47] UFL Access reported that Harrell would work out in front of United Football League representatives in Las Vegas.

He hosted a weekly sports talk show called the "Witt Ford Red Zone with Graham Harrell" every Monday during the 2012 season on 104.1 FM WRLU and archived at DoorCountyDailyNews.com.

Harrell played for one series at the end of the Jets' final 2013 preseason game against the Eagles in relief of Matt Simms.

Harrell helped tutor the OSU quarterbacks with the new Air Raid system that Gundy had hired Holgorsen to run.

[60] In February 2015, Harrell was named the football program's outside receivers coach, replacing Dennis Simmons.

Despite being named OC at North Texas, Harrell said he would honor his contract with Leach and WSU and coach the Cougars in the Sun Bowl.

At UNT, Harrell inherited an offense that had ranked in the bottom 25 of the country in passing, total yards, and points in 2015.

[64] On January 10, 2022, Harrell was hired as the offensive coordinator and quarterbacks coach at West Virginia University.

[66] On September 29th, 2024, it was announced that Harrell was fired from his position as offensive coordinator after the team started the season 1-3 while averaging only 12.7 points per game during their 3 losses.

Graham also has a younger brother, Clark Harrell, who played a year at quarterback for the Tulsa Golden Hurricane.

Sam Harrell, Graham's father, was the highest paid high school coach in Texas as of the 2005–06 academic year.