Born in Missoula, Montana, Friesz moved with his family in 1975 to Coeur d'Alene, Idaho. He attended Coeur d'Alene High School and graduated in 1985. Friesz spent those seasons as a back-up and became the starter in his senior season in the fall of 1984. His first game was against perennial state power Borah in Boise, who had beaten the Vikings 27-23 at Coeur d'Alene in 1983. The #2-ranked CDA Vikings returned the favor and beat the Lions 19-14 on the then-green AstroTurf of Bronco Stadium. Friesz completed 21 of 40 passes for 260 yards and two touchdowns in his starting debut. The Vikings took over the top spot in the state poll and beat Rogers of Spokane 46-0 the following week. They won their first ten games, but lost in the state semifinals 25-20 to Capital, who also had handed them their last loss, at the finals in 1983. The game was played at a neutral site, the Kibbie Dome in Moscow, Friesz's future home field. He finished the season with over 1,900 yards passing and 19 touchdowns in eleven games.
Friesz was the Vandals starting quarterback for three years, beginning in 1987, when he threw 28 touchdown passes as a sophomore and was named player of the year in the Big Sky and second team All-American (Division I-AA). In his junior season of 1988, he guided the Vandals to a 9-1 regular season and two playoff wins, ending the season with a road loss in the Division I-AA semi-finals. He was a consensus All-American selection at quarterback.
In his senior season in 1989, Freisz threw 31 touchdowns and for over 4,000 yards in guiding the Vandals to their third consecutive conference championship. Idaho went undefeated in conference play (8-0), the only time in school history. Friesz averaged over 360 yards per game and passed for over 300 yards in ten consecutive games. He received the Walter Payton Award as the outstanding player in the nation in Division I-AA.
In his college career, #17 passed for over 10,000 yards and was the conference player-of-the-year for three consecutive years. The Vandals' annual MVP award has been renamed the John Friesz Award in his honor.
Friesz became the starting quarterback for the Chargers in 1991, his second season. He suffered a season-ending knee injury in a pre-season game in 1992. The Chargers then acquired QB Stan Humphries from the Washington Redskins, who was drafted by then Redskins GM Bobby Beathard, who was the Chargers GM at that time. Humphries became the Chargers starting QB and led the Chargers who were 0-4 to start the 1992 season to an 11-5 record, helping the Chargers end a 10-year playoff drought and winning their first AFC West Division title since 1981 under rookie coach Bobby Ross. Friesz would return to the Chargers the next season as the backup QB and left the Chargers as a free agent prior to the 1994 season.
Friesz passed for over 8,600 yards and 45 touchdowns in his professional career.