As a high school football quarterback, Fassel played for the legendary Anaheim High School coach Clare Van Hoorebeke for whom his father was the equipment manager. Fassel and tight end Gerry "Moon" Mullins (a future standout offensive lineman for the Pittsburgh Steelers) led the Colonists to the 1967 CIF Southern Section large schools football championship, winning the title game against Sunset League rival Santa Ana High School in Anaheim Stadium. The Saints featured future Cal-Berkeley and Cincinnati Bengals star running back Isaac "Bobby" Curtis. It was the first time that two teams from the same league played for the CIF Southern Section championship.
Fassel played quarterback at Fullerton College, later at California State University, Long Beach after playing at USC in 1969. He was drafted in the 7th round by the Chicago Bears in the 1972 NFL Draft. He played briefly with The Hawaiians of the WFL in 1974, and became an assistant coach during the 1974 WFL season when the team brought New York Giants quarterback Randy Johnson in from the NFL. He left the WFL after the '74 season, and was working in air conditioning in Utah when the Hawaiians needed a quarterback late in the 1975 season. He played in the final game of the WFL for the Hawaiians, throwing the last pass in the league's history as the WFL folded three days later on October 22, 1975. Jim Fassel's total passing stats with the Hawaiians for 1974 and 1975 were 16 completions out of 39 attempts for 205 yards. He did not throw any touchdowns and was intercepted four times.
During Fassel's time as Giants head coach, his teams were known for numerous post-season runs in December and for winning big games, such as against the previously undefeated Denver Broncos in 1998. In 1997, he was named NFL coach of the year. He resurrected the career of quarterback Kerry Collins and received acclaim for his "playoff guarantee" in the 2000 season, during which he led the Giants to an improbable Super Bowl appearance.
However, his legacy as head coach for the Giants is mixed. Fassel's Giants were known for their disappointments against inferior teams in the regular season, as well as in the playoffs. The most notable loss was a shocking 39-38 loss to the San Francisco 49ers in the 2002 postseason, in which they blew a 38-14 third quarter lead. During the 2003 season, injuries decimated the Giants and he was fired amidst some controversy.
Critics of Fassel have pointed to his lack of success as offensive coordinator after two seasons with the Ravens, in 2004 and 2005. During that time, the Ravens ranked near the bottom of the league in offense.
On October 17, 2006, Fassel was fired as offensive coordinator for the Ravens.
It is believed that Fassel had been in contention for the Washington Redskins head coaching position in 2004, but when former Redskins coach Joe Gibbs agreed to come out of retirement, team owner Dan Snyder hired the Hall of Fame three-time Super Bowl winner.
On January 23, 2008, it was reported in Newsday.com that Jim Fassel was again the leading candidate for the Redskins' head-coaching position, which opened up as a result of Joe Gibbs' retirement for the second time in that position. However, on February 9, 2008, Jim Zorn was hired as the new Redskins' head coach.
In November 2008, Fassel reportedly sent a letter to Oakland Raiders owner Al Davis expressing interest in becoming the next head coach of the Raiders.
Fassel returned to the Locos in 2010 and repeated as champions, again defeating the Tuskers in the 2010 UFL Championship Game. The Locos tried to three-peat in 2011, but this time fell to the Tuskers (who had since been relocated and renamed the Virginia Destroyers) in the 2011 UFL Championship Game. Fassel was the only current UFL head coach who was active in the league since its inauguration and was the Locos' head coach when the league suspended play in 2012.
Fassel entered broadcasting following his firing as offensive coordinator for the Ravens, joining Westwood One radio as a color commentator for its Sunday NFL action. He stayed with the network for two seasons, calling Sunday afternoon games with Harry Kalas in 2007 and Sunday Night Football with Dave Sims. Fassel was also part of Westwood One's playoff coverage those two years, calling various games, and worked the 2007 and 2008 NFC Championship Games with Bill Rosinski (2007) and Marv Albert (2008).
Before the 2003 NFL season, Fassel was reunited with a son, John Mathieson, whom he and his wife Kitty gave up for adoption in 1969. The couple was unmarried when he was born.
In 2004 and 2005, Jim and Kitty Fassel owned a Quiznos restaurant in Denville, New Jersey. Their son Brian was in charge of day-to-day operations. The store was sold in April 2005 but has since closed.
Jim and Kitty divorced in 2006 after years of counselling.
^Anderson, Dave. "Sports of The Times; Fassel's Finished Basement", The New York Times, March 2, 2001. Accessed November 2, 2007. "Maybe that explains how the Fassels celebrated when he returned to their Ho-Ho-Kus, N.J., home on Tuesday with a four-year, $10.75 million contract -- a guarantee that they will be living at the same address for at least eight years, their longest consecutive residence."