Martz is known for creating complicated offensive schemes, particularly in the passing game. He developed Kurt Warner and Marc Bulger into two of the league's finest quarterbacks of recent years.

Martz played tight end at San Diego Mesa College, the University of California, Santa Barbara, and Fresno State University, and graduated summa cum laude from Fresno State in 1972. The following year his coaching career began at Bullard High School in Fresno, California. From 1974 to 1991, he was an assistant coach at seven colleges and universities, including two stints as offensive coordinator at Arizona State University.

From 1992 to 1996, he was a tight ends, receivers, and quarterbacks coach for the Los Angeles Rams. In 1997 and 1998, he was the quarterbacks coach for the Washington Redskins, and helped develop untouted Trent Green into a promising NFL quarterback.

In 1999, Martz was hired as the offensive coordinator of the St. Louis Rams, who went on to win Super Bowl XXXIV that season. Martz was named head coach of the Rams on February 2, 2000 after Dick Vermeil (temporarily) retired. He led the Rams to a 10-6 regular season record, but they lost in the 1st round to the New Orleans Saints 28-31. In 2001 the Rams went 14-2 and made it all the way to Super Bowl XXXVI, only to lose to the New England Patriots 20-17 in a major upset. According to a book written about New England coach Bill Belichick, one of the key factors in the upset was Martz's refusal to abandon his beloved passing attack late in the game as the Rams were attempting to run out the clock, instead telling his players he wanted to finish off the Super Bowl season "(his) way." It is notable, however, that the Rams' two touchdowns in the second half came when they trailed the Patriots and were forced to nearly abandon the running game. Running back Marshall Faulk had just five carries in the entire second half, in which the Rams outscored the Patriots 14-6. In 2002 the Rams had a see-saw season in which Kurt Warner played injured and committed more turnovers than usual. In 2003, Marc Bulger's first full year as a starter, the Rams lost in overtime in the NFC divisional playoff to Carolina, missing out on a chance to go back to the Super Bowl. In 2004, after a slow start, the Rams finished strong, and with an 8-8 record snuck into the playoffs tied for the worst record in NFL playoff history. They had to overcome an unusual amount of injuries. They still managed to beat the Seattle Seahawks in the first round, but got clobbered by the Atlanta Falcons in the divisional round. The loss of defensive coordinator Lovie Smith, who left for Chicago and took two of his assistants, clearly hurt the team.

On October 10, 2005, Martz took a leave of absence from the Rams to treat a persistent bacterial infection in his heart. Assistant head coach Joe Vitt handled coaching duties as the interim head coach for the rest of the season and offensive coordinator Steve Fairchild served as the play-caller. Martz told the Rams that after being examined and evaluated by his treating physician, Dr. Victoria Fraser, that his illness would prevent him from performing his duties. Martz immediately announced he would miss the rest of the season.

Martz, known for being tightly wound and idiosyncratic, had several conflicts with the St. Louis front office over the years which came to a climax in 2005. While recovering from his illness at home and watching a live Rams game on television, Martz was blocked by team president John Shaw from relaying a play call to Fairchild by phone. He continued to show up periodically at team practices late in the season, and was given medical clearance to coach the Rams' last regular season game on New Year's Day; instead, he showed up at the Rams training facility on December 28, 2005 and reportedly accused two of his coaches of disloyalty in his absence, then the next day told his entire staff to take the next week off. As the media craze swirled, the anti-Martz sentiment rose in St. Louis. When sites like and FireMartz.com became more and more popular, it was apparent that Martz's days in St. Louis were numbered. As expected, Martz was dismissed by the Rams on January 2, 2006.

During the seven years in which Martz was involved with the Rams, the team went to two of the franchise's three Super Bowls.

Martz interviewed for head coaching vacancies in Oakland and New Orleans. After the interview, he withdrew his candidacy for the position in Oakland. After initially rejecting an offer due to financial considerations, on February 8, 2006, Martz accepted and offer from the Detroit Lions to be their offensive coordinator and their QB coach. He is expected to turn around one of the worst offenses in the NFL over the past five seasons and get the most out of what many believe is a potentially explosive offense.