After leaving Kansas City, Cassel had a brief stint with the Minnesota Vikings before spending the rest of his career as a backup for the Buffalo Bills, Dallas Cowboys, Tennessee Titans, and Detroit Lions.

When he was 11 years old, his family home was at the epicenter of the 1994 Northridge earthquake, which at one point trapped his father Greg under a marble column and caused water from their swimming pool to flood their house.

[3] Cassel attended Chatsworth High School and was a letterman, an all-city selection, and a standout in football, basketball, and baseball.

[7] Cassel also had an appearance in the HBO Family program Freshman Year, a reality show in which his younger brother was one of the featured students.

As a result of Leinart's success, Cassel spent time at tight end and wide receiver in 2003, and some special teams that year.

[16] Cassel, a communication major at USC, was also roommates with Pittsburgh Steelers safety Troy Polamalu[17] and Carson Palmer.

[24] Cassel began the 2005 season third on the Patriots' depth chart behind Tom Brady and Doug Flutie, after beating out two more veteran quarterbacks, Chris Redman and Rohan Davey.

He saw his first regular season action in the closing minutes of the Patriots' 41–17 loss to the San Diego Chargers on October 2, 2005, going 2-for-4 for 15 yards and an interception.

Though he was sacked for a safety, Cassel threw two touchdowns, one to wide receiver Tim Dwight, and the second to tight end Benjamin Watson.

Although the Patriots considered signing a veteran quarterback to compete with him, Cassel played well in preseason and became the primary backup to Brady.

[27] Cassel was on the 45-man active roster for all 16 games in 2006; when the Patriots brought in yet another Heisman winner, Vinny Testaverde, who acted as the emergency quarterback.

[28] After injuries to Josh Miller and Ken Walter in Week 16, Cassel assumed duties as holder for kicker Stephen Gostkowski.

[35] The Patriots' 19–10 victory was the first time in six tries that a quarterback making his first NFL start defeated a team led by Brett Favre.

[43][44] During Week 15 against the Oakland Raiders, Cassel, playing just six days after the death of his father, set a new personal best, throwing for four touchdowns in a 49–26 victory.

Cassel, playing in snow for the first time ever,[46] nevertheless completed 20 of 36 passes for 345 yards, three touchdowns, and no interceptions, while helping the Patriots remain in the hunt for the AFC East title.

On February 28, 2009, the Patriots traded both Cassel and outside linebacker Mike Vrabel to the Kansas City Chiefs for the 34th overall selection (Patrick Chung) in the 2009 NFL draft.

In an interview, Cassel said he would "thank" Pollard for the opportunity, but also stated that he believed the hit on Brady was "unintentional" and he never wishes to see anyone get hurt while playing.

[67] As a result of his performance against the Seattle Seahawks, Cassel won the FedEx Air & Ground NFL Players of the Week award, as voted by the fans.

Cassel returned for the next two games however and led the Chiefs to landslide wins over both the St. Louis Rams in the Governors Cup and then at home against the Tennessee Titans.

[98][99] Cassel was involved in a three-way quarterback competition between former Bills first round pick EJ Manuel and former Ravens backup Tyrod Taylor.

[101] Cassel was credited as the Bills' starting quarterback for the season opener due to him taking the team's first snap on offense in the wildcat formation with Taylor lined up at wide receiver.

On December 19, after a disastrous showing against the New York Jets, Cassel was benched and replaced by Kellen Moore for the game and the rest of the season.

[106] On Cassel's last play before being benched for Moore, he became the first quarterback in over twelve years to throw an interception that was also flagged for intentional grounding.

[107] The play was designed to be a wide receiver bubble screen, but Cassel held onto the ball after a pump-fake, then turned to look at the other side of the field.

Forced to scramble to get away from the immediate pressure, thanks largely to most of the Dallas offensive linemen moving to the left side of the field to block for the presumed wide receiver screen, Cassel stumbled and nearly fell over.

[109] During a Week 16 38–17 road loss to the Jacksonville Jaguars on Christmas Eve, Cassel entered the game in the third quarter after starter Marcus Mariota suffered a leg injury, completing 13-of-24 passes for 124 yards, a touchdown, and an interception.

[110] Due to Mariota's injury, Cassel started in the regular-season finale against the Houston Texans, completing 16-of-26 passes for 150 yards, a touchdown, and an interception as the Titans won 24–17.

[113] During a Week 4 57–14 road loss to the Houston Texans, Mariota suffered a hamstring injury in the third quarter and was relieved by Cassel.

[114] Due to Mariota's injury, Cassel started the game against the Miami Dolphins on October 8, completing 21 of 32 passes for 141 yards and a touchdown as the Titans lost on the road by a score of 16–10.

[117] During the season opener against the New York Jets, Cassel made his Lions debut coming in relief of Matthew Stafford in the 48–17 blowout loss.