Dennis attended Southwood High School in Shreveport, Louisiana, where he played as a quarterback and defensive back. As a senior, he received All-state honors at quarterback.
He accepted a football scholarship from the University of Louisiana at Monroe, where he was a three-year starter at cornerback. As a freshman, he registered 65 tackles, 3 interceptions and 8 passes defensed. The next year, he led the nation with 7 interceptions (school record), while tallying 18 passes defensed (school record) and 75 tackles. As a junior, he made 40 tackles, 3 interceptions and 10 passes defensed.
He declared for the NFL Draft after his junior season, finishing his college career with 180 tackles, 13 interceptions (tied for third all-time) and 36 passes defensed (school record).
Dennis was selected by the Kansas City Chiefs in the fifth round (162nd overall) of the 2000 NFL Draft. As a rookie, he started the final 13 games at left cornerback in place of an injured James Hasty, while registering 70 tackles, 2 fumble recoveries, one forced fumble and one interception.
In 2001, he suffered a sprained medial collateral ligament in his right knee during the third pre-season game against the Jacksonville Jaguars. On September 2, he was placed on the injured reserve list. He was waived injured on October 2.
On October 3, 2001, he was claimed off waivers by the Dallas Cowboys. He appeared in 11 games in the team's nickel defense after recovering from his right knee injury, posting 22 tackles, 7 passes defensed and 8 special teams tackles. He was released on September 1, 2002.
On September 2, 2002, he was claimed off waivers by the Houston Texans, becoming a part of the franchise inaugural season. He suffered a torn ACL ligament in the sixth game of the year against the Cleveland Browns. On October 22, he was placed on the injured reserve list, finishing with 5 special teams tackles and one defensive tackle.
On June 18, 2003, he had to undergo a second knee surgery on the same knee he tore the ACL ligament. On August 26, he was placed on the injured reserve list. He was released April 27, 2004.
On October 13, 2004, he was signed as a free agent by the Washington Redskins after safety Matt Bowen was lost for the season. He wasn't re-signed at the end of the year.