He played college football for the Oregon Ducks, where he earned Pac-10 Offensive Player of the Year as a senior, and was selected third overall by the Detroit Lions in the 2002 NFL draft.

He spent his final three seasons as the primary starter for the Miami Dolphins and Atlanta Falcons and a backup with the New Orleans Saints.

[5] Harrington's best collegiate game was arguably the 2002 Tostitos Fiesta Bowl at Sun Devil Stadium in Tempe, Arizona when he threw for 350 yards and four touchdowns and helped lead the Ducks to a 38–16 victory over Colorado.

Front office mismanagement, woeful offensive line protection, lack of talent at other skill positions, and an erratic philosophical change in the team's identity to a conservative West Coast offense-oriented attack under head coach Steve Mariucci may have played a factor in Harrington not realizing his potential professionally, as well as his own play and lack of talent.

During the 2003 season, Harrington joked that being "the quarterback of the Lions and the goalie of the Red Wings" were the hardest jobs in Detroit.

Harrington regained the starting role the week after Garcia threw a game-ending interception returned for a touchdown in overtime against Chicago.

Harrington started again for Detroit on November 13, 2005, against the Arizona Cardinals, throwing for three touchdowns without an interception in the Lions' 29–21 win.

Culpepper injured his shoulder prior to Miami's fifth game against the New England Patriots, forcing Harrington into the starting role.

Harrington capped off this winning streak in front of a national television audience on Thanksgiving Day in Detroit with a 27–10 victory at Ford Field against his former team.

Harrington was pulled midway through Miami's next game against the New York Jets, replaced in the 13–10 Christmas night loss by Cleo Lemon.

On April 9, 2007, Harrington agreed to a two-year, $6 million contract with the Atlanta Falcons to compete with D. J. Shockley and Chris Redman to be the backup quarterback to Michael Vick.

During the Week 3 Atlanta home opener against the division rival Carolina Panthers, Harrington completed 31-of-44 passes with two touchdowns and no interceptions for a 110.1 passer rating in a 27–20 loss.

In Week 4, Harrington improved on his numbers with a 121.7 passer rating, completing 23-of-29 passes for two touchdowns with no interceptions, leading the Falcons to their first win of the 2007 season.

Harrington was first given the label of "Savior" by fans and media in Detroit - then deemed a "bust" when he did not meet high expectations.

Many speculate that his premature start in the NFL, along with lack of surrounding talent, poor coaching, and questionable offensive lines have affected his performance severely.

Many other quarterbacks, such as Tim Couch and David Carr, were also drafted highly and eventually lost their starting jobs.

"[26] Former defensive end Howie Long said that Millen made a mistake by drafting Harrington, along with signing Garcia instead of Brad Johnson.

Emily is a nurse practitioner, and Harrington spoke about them opening a medical clinic to serve the homeless in Portland, after he retired from football.

[27] Harrington is an accomplished jazz pianist who has occasionally performed with artists such as Jason Mraz, Blues Traveler, and Third Eye Blind.

[28] On February 1, 2008, Harrington appeared as a guest chef on a special Super Bowl episode of The Rachael Ray Show.

After being given the New York Times Square "Joey Heisman" billboard by the former Oregon Ducks Athletic Director Bill Moos, he proceeded to cut it up and sell the pieces for charity.