He started his college career as a linebacker for Arizona State University in 1994, when he secured the last remaining scholarship for the team. Tillman excelled as a linebacker at Arizona State, despite being relatively small for the position at five-feet eleven-inches (1.80 m) tall. As a junior, he helped his team go undefeated that season as well as helping them make it to the Rose Bowl that year. In 1997, he was voted the Pac-10 Defensive Player of the Year. Academically, Tillman majored in marketing and graduated in three and a half years with a 3.85 GPA. He also earned many academic awards including: the Clyde B. Smith Academic Award in 1996 and 1997; the Sporting News Honda Scholar-Athlete of the Year in 1997; and the 1998 Sun Angel Student Athlete of Year. He was inducted into the College Football Hall of Fame in 2010.
In the 1998 National Football League Draft, Tillman was selected as the 226th pick by the Arizona Cardinals. Tillman moved over to play the safety position in the NFL and started ten of sixteen games in his rookie season.
At one point in his NFL career, Tillman turned down a five-year, $9 million contract offer from the St. Louis Rams out of loyalty to the Cardinals.
Sports Illustrated football writer Paul Zimmerman (Dr. Z) named Tillman to his 2000 NFL All-Pro team after Tillman finished with 155 tackles (120 solo), 1.5 sacks, 2 forced fumbles, 2 fumble recoveries, 9 pass deflections and 1 interception for 30 yards.
Tillman finished his career with totals of 238 tackles, 2.5 sacks, 3 interceptions for 37 yards, 3 forced fumbles, 2 pass deflections, and 3 fumble recoveries in 60 career games. In addition he also had 1 rush attempt for 4 yards and returned 3 kickoffs for 33 yards.
In May 2002, eight months after the September 11 attacks and after completing the fifteen remaining games of the 2001 season which followed the attacks (at a salary of $512,000 per year), Tillman turned down a contract offer of $3.6 million over three years from the Cardinals to enlist in the U.S. Army.