Karl attended Helix High School in La Mesa, California, where he played football. He was a two-time all-league selection and an honorable mention All-American as a senior. He led Helix to the CIF San Diego Section second place in 1981. During the 1983 season, he was a teammate of quarterback Rick Neuheisel, who would be his eventual successor as UCLA head coach. He caught touchdowns from Neuheisel during the season, including two in the 1984 Rose Bowl. He had a brief career as a player in the NFL with the Dallas Cowboys in the 1987 season, but he was placed on the injured reserve.