Wagner attended Colony High School in Ontario, California, and was a Titans high school football player. As a senior, he had 125 tackles (92 solo, 33 assisted), including four sacks. He finished his career with 250 tackles (184 solo, 66 assisted) and seven interceptions. He went on to play for the University of California, Berkeley where he started 11 games as a freshman before being sidelined by an ankle injury. As a sophomore, he played in all 12 games, starting six at outside linebacker. He led the team with 81 tackles (7 for loss) and also had three pass defenses. As a junior, he started all 12 games and was named second-team All-Pacific Coast Conference. He finished with 69 tackles (6 for loss) and two forced fumbles. After his junior year, Wagner entered the 1997 NFL Draft. He was selected third overall by the New York Giants and began his career with them that same year. He appeared in 15 games over two seasons with the Giants and recorded 35 tackles (3 for loss). In 1999, he joined the St. Louis Rams and started 13 games at outside linebacker. He had 57 tackles (5 for loss) and two passes defensed.