Reich was accepted to the University of Maryland on a football scholarship and played backup to Boomer Esiason the first few years. The biggest highlight of his college career was the comeback he led against the Miami Hurricanes on November 10, 1984 at the Orange Bowl Stadium. Reich came off the bench to play for Stan Gelbaugh, who had previously replaced him as the starter after Reich separated his shoulder in the fourth week of the season against Wake Forest. Quarterback Bernie Kosar had led Miami to a 31–0 halftime lead. At the start of the third quarter, Reich led the Maryland Terrapins on multiple scoring drives. Three touchdowns in the third quarter and a fourth at the start of the final quarter turned what was a blowout into a close game. With Miami leading 34–28, Reich hit Greg Hill with a 68-yard touchdown pass, which deflected off the hands of Miami safety Darrell Fullington, to take the lead. Maryland scored once more to cap a 42–9 second half, and won 42–40, completing what was then the biggest comeback in NCAA history.
Reich was drafted by the Buffalo Bills in the third round (57th overall) in the 1985 NFL Draft. The Bills already had drafted future Hall of FamerJim Kelly in 1983 and when Kelly signed with the Bills in 1986, Reich was relegated to the backup role. Reich got his first start when Kelly went down with a shoulder injury in 1989. Reich led the Bills to two straight victories. He rallied the Bills in the fourth quarter by throwing two drives down the field for a 23-20 victory over the previously unbeaten Los Angeles Rams. This first game for Reich occurred in front of a Rich Stadium crowd of 76,231 and a Monday Night Football audience. Reich returned the following season, however, when Kelly was injured again late in the season. Reich provided the Bills with two key wins, clinching them the AFC East title and home field advantage throughout the playoffs. Reich is one of a handful of quarterbacks who is undefeated as a starter in post-season play, but he is the only one with more than one start to his credit.
On January 3, 1993, in a wild card playoff game, the Houston Oilers led the Bills 35-3 early in the 3rd quarter. Reich then led the Bills on a 38-3 run en route to a 41-38 overtime victory, the largest comeback in NFL history. Reich would help the Bills defeat the Pittsburgh Steelers in the divisional round with a score of 23-7 before once again giving the team back to Kelly, who led them into Super Bowl XXVII, where they fell to the Dallas Cowboys 52-17. Reich again replaced an injured Kelly in the first half of the Super Bowl, and led the Bills to 10 points, though a touchdown came on an illegal pass across the line of scrimmage that wasn't called. However, the game was already well out of hand, and he finished with two interceptions.
After giving the Bills one more comeback victory late in the 1993 NFL season, Reich was picked from the Bills in the 1995 expansion draft to join the Carolina Panthers to start off their first year. He threw the first touchdown pass to former Bills player Pete Metzelaars in Ericsson Stadium (currently Bank of America Stadium). The Panthers had drafted Kerry Collins as their intended franchise quarterback, but Reich was the starter for the first three games until Collins was deemed ready to take the starting job. He was signed by the New York Jets where he started for seven games in 1996. In 1997 Reich signed with the Detroit Lions, reuniting him with his coach at Maryland, Bobby Ross. Reich appeared in 6 games in 1997, all in relief, and 6 games in 1998, including 2 starts. Reich retired following the 1998 NFL season.
In 2014, NFL Hall of Fame Executive Bill Polian, who drafted Frank Reich, called him "the greatest backup quarterback in NFL history."
In 2008, he served as an offensive assistant and quarterbacks coach for the Indianapolis Colts. After Tony Dungy retired early in 2009, Jim Caldwell (former Colts quarterback coach) replaced Dungy and Reich took his place as the new quarterbacks coach. Reich switched to Wide Receivers Coach in 2011 but was dismissed when the entire coaching staff was released. He was the wide receivers coach for the Arizona Cardinals in 2012 under head coach Ken Whisenhunt, but both Reich and Whisenhunt (along with other offensive coaches) were dismissed on December 31, 2012. He was hired by the San Diego Chargers along with Whisenhunt in 2013. When Whisenhunt left to become head coach of the Tennessee Titans, Reich was promoted to offensive coordinator.
Throughout Reich's NFL career, he remained a devout Christian. He is a motivational speaker utilizing the great comebacks and the importance of God as a main keynote of his speeches. He credits the song "In Christ Alone" by Michael English as his inspiration. He belongs to the Premier Speakers Bureau where his main topics are communication and teamwork. Reich attended the Charlotte Campus of Reformed Theological Seminary, Charlotte, North Carolina, where he earned a Master of Divinity degree. He served as president of RTS' Charlotte campus from 2003-2006. Reich was also a pastor at Ballantyne Presbyterian until he moved to Indianapolis.
While living in Buffalo, New York in the early 90s, Reich met a man named Jim Case whom he hired to build his deck and refinish his basement. A friendship between Reich and Case formed and they started a company together called Pro Display Inc. which specialized in sports memorabilia. Their main item was a football stand inspired by Reich's father, who created a wooden stand shaped like a football field where the goal posts held up the football. Reich had co-founded a retail business in the 90s that he started with friend Boomer Esiason and two others. They were part owners of Jim and Chuck's Boot Shop. The store expanded over the New England area with fifteen stores where they still remain today.