[3][4] Clark has a sickle cell trait condition which made it dangerous for him to play at high altitudes, such as in Denver, Colorado.

While at LSU, he joined the Nu Iota chapter of Kappa Alpha Psi Fraternity, Inc. Clark went undrafted in the 2002 NFL draft.

[7][8] During training camp, Clark delivered a late hit to teammate and rookie wide receiver Tim Carter that sidelined the second round pick for six weeks.

Head coach Jim Fassel named Clark a backup safety and a part of the special teams unit to begin his rookie season.

Head coach Jim Fassel named Clark the primary backup safety to begin the regular season, behind starters Shaun Williams and Omar Stoutmire.

On October 19, 2003, Clark earned his first career start and deflected a single pass during a 14–10 loss against the Philadelphia Eagles in Week 7.

On December 28, 2003, Clark recorded three solo tackles and made his first career sack during a 37–24 loss against the Carolina Panthers in Week 17.

[7] Head coach Joe Gibbs named Clark a backup safety behind Matt Bowen, Andre Lott, and rookie Sean Taylor.

Clark finished the 2005 NFL season 57 combined tackles (42 solo), a career-high three interceptions, two pass deflections, two forced fumbles, and was credited with half a sack in 13 games and 13 starts.

On October 8, 2006, Clark recorded nine combined tackles (six solo), one pass deflection, and made his first interception as a member of the Pittsburgh Steelers during a 23–12 loss at the San Diego Chargers in Week 5.

During a 2007 game against the Denver Broncos at Invesco Field at Mile High, Clark developed severe pain in his left side and had to be rushed to the hospital.

It turned out that Clark had had a splenic infarction due to the sickle cell trait[5] which he has had since he was a child; these are usually a risk at high altitudes.

[26] After the 2008 season, Clark started in Super Bowl XLIII where the Steelers won against the Arizona Cardinals, recording five total tackles.

[27] After the 2010 season, he started in the Steelers' appearance in Super Bowl XLV against the Green Bay Packers, where he recorded eight total tackles in the 31–25 loss.

[29] The Steelers proceeded with their plans to replace him after they signed Mike Mitchell to a long-term contract on March 11, 2014, to become the team's new starting free safety.

He joined forces with students from the Leadership In Public Services High School located at Ground Zero for "Operation PaintFest."