After graduation from high school in 2004, O'Connell enrolled at San Diego State University and played quarterback for the Aztecs, starting for three seasons.

He played 5 years as a quarterback in the NFL, with New England in 2008, the Detroit Lions in 2009, the New York Jets from 2009 to 2011, the Miami Dolphins and the San Diego Chargers in 2012.

O'Connell's NFL coaching career began in 2015; he held various assistant coaching roles on the Cleveland Browns, San Francisco 49ers, Washington Redskins, and Los Angeles Rams (where he won Super Bowl LVI as offensive coordinator, in the 2021–2022 season).

Kevin was born in Tennessee, the family moved to New York in 1990, and eventually to Carlsbad, California up the coast about 35 miles from San Diego.

[12] As a true freshman in 2003, O'Connell, age 18, and sophomore Matt Dlugolecki were backups to Adam Hall who was coming off a strong junior season in 2002.

[14] O'Connell did not see any game action in 2003, registered no statistics, and retained four years of eligibility under the NCAA's redshirt rule.

[15] While O'Connell was touted as the team's "stronger armed quarterback,"[16] he spent the 2004 season as San Diego State's No.

[22] After the loss to BYU, O'Connell led San Diego State to victories over Air Force (37–31) and UNLV (21–3).

[23][24] In the spring of 2005, O'Connell was in a three-way competition for San Diego State's starting quarterback job.

"[25] In the 2005 spring game, O'Connell didn't participate due to his recovery from shoulder surgery, and gave Dluglocecki, Mougey and Kevin Craft opportunities to take all of the repetitions.

[26] O'Connell was the starting quarterback in all 12 games for the 2005 San Diego State Aztecs football team that compiled a 5–7 record.

[32] In the spring of 2006, O'Connell was in a three-way competition for San Diego State's starting quarterback job again.

"[35] In his first start of the season, O'Connell and the Aztecs lost 34–27 a UTEP Miners team that was led by Jordan Palmer.

[36] In relief of starting quarterback, Kevin Craft, O'Connell lead the Aztecs to a victory over UNLV.

[47] During the 2008 season; playing under head coach Bill Belichick, O'Connell rushed for a touchdown in the 2008 preseason against the New York Giants in the fourth quarter, in a game the Patriots eventually lost 19–14.

[48] O'Connell made his NFL debut on September 21, 2008, in the fourth quarter against the Miami Dolphins, throwing his first four career passes and completing three.

[50] Going into the 2009 season, O'Connell was competing with veteran quarterback Andrew Walter, signed after his release from the Oakland Raiders, and undrafted free agent rookie Brian Hoyer of Michigan State, for the backup position behind Tom Brady.

On September 6, 2009, the Lions traded O'Connell to the New York Jets for a seventh round 2011 draft pick.

[57] O'Connell then found that he had a torn labrum in his throwing arm, an injury he had received during the preseason, which required surgery.

After spending the previous two years as a private quarterbacks coach in Carlsbad, California with clients including Johnny Manziel, Logan Thomas, Marcus Mariota, and Bryce Petty, it was announced on February 17, 2015, that O'Connell was hired as the quarterbacks coach of the Cleveland Browns.

That began his full-time coaching career and reunited him with Mike Pettine, who was the defensive coordinator for the Jets during O'Connell's stint as a player there, as well as Manziel, who was a client of O’Connell’s prior to the 2014 NFL Draft.

[72][73] On February 16, 2022, just three days after winning Super Bowl LVI, O'Connell was hired by the Minnesota Vikings as the 10th head coach in their franchise history.

O'Connell won his first game as head coach against the Green Bay Packers in Week 1 of the 2022 season by a score of 23–7.

[76] O'Connell led the Vikings to a 13–4 record on the season, tying Matt LaFleur and Jim Harbaugh for the second-most wins for a rookie head coach in NFL history, which included an 11–0 record in games decided by one score, including the biggest comeback in NFL history.

After Kirk Cousins left the Vikings for the Atlanta Falcons after the season, O'Connell worked closely with veteran free agent quarterback Sam Darnold and rookie first round pick J.J. McCarthy in the 2024 off-season.

A preseason knee injury to McCarthy complicated the team's plans for quarterback progression, leading to Darnold becoming the sole starter for 2024.

O'Connell guided the Vikings to a 14–3 record, the second most wins in franchise history, with both Darnold and newly acquired running back Aaron Jones registering career highs in passing and rushing yards, respectively.

However, due to a Week 18 loss to the Detroit Lions, the Vikings became the first 14-win team to settle for a Wild Card round berth, finishing second in the NFC North.

In the Wild Card round, O'Connell matched up against his former mentor Sean McVay for a second time.

The Vikings would lose to the Los Angeles Rams 27–9 in a game played at State Farm Stadium in Arizona due to the wildfires in southern California.