Demps was a walk-on who was a two-year starter at San Diego State University where he was also a member of Sigma Pi fraternity. He was a two-time All-Mountain West first-team selection. During his college career, Demps recorded 229 career tackles, including 15 tackles for loss, and five interceptions. Demps played primarily on special teams as a red-shirt freshman. He majored in Information Decision Systems. Demps posted 140 tackles and one interception as a sophomore in 1999. He recorded his first career interception and returned it 73 yards for a touchdown against Wyoming. Demps also had five tackles, including one tackle for a loss against the Cowboys. Demps started all 11 games at strong safety as a junior. He earned first-team All-Mountain West honors and finished second on the team and sixth in Mountain West with 97 tackles. He also had two interceptions and led the team with 15 tackles for loss. He opened the season with 15 tackles, including three tackles for a loss. Demps started all 11 games at strong safety as a senior in 2001 and earned a second consecutive First-team All-Mountain West honors. He posted 92 tackles, including a career-high 59 solo, along with two interceptions.
Professional career
Baltimore Ravens
Demps went undrafted in the 2002 NFL Draft, but was signed by the Baltimore Ravens where he played from 2002 to 2005. He played in 14 games, starting 10 after making the Ravens roster as a rookie free agent. He recorded 53 tackles, one sack, one forced fumble and a game-saving interception vs. Jacksonville. In 2003, Demps saw action in all 16 regular season games with nine starts for Ravens and recorded 59 total tackles, two interceptions and one forced fumble. He made his first career post-season start in Baltimore's Wild Card loss to Tennessee. In 2004, Demps started all 16 games for the first time in his career, tying for sixth on the team with 86 tackles, including 72 solo. He also recorded 2.5 sacks, two forced fumbles, two fumble recoveries and one interception on the season. Demps notched career highs with 15 tackles, including 14 solo against the Bengals. Demps started in all 11 games in which he played at free safety for Baltimore in 2005 before tearing his ACL and being placed on injured reserve on December 2, 2005. Before his injury, he tallied 70 tackles and two fumble recoveries, plus seven special teams tackles.
New York Giants
Demps was signed by the New York Giants in the offseason. He started all 16 games for the Giants in 2006.
Houston Texans
Demps was signed by the Houston Texans on September 11, 2007. He played in 15 games, starting the last 8 games of the season. He was named an AFC Pro Bowl alternate. The Texans released Demps on February 25, 2009.