[1] Williams played college football for the Auburn Tigers, where he set the school's record for career rushing touchdowns and helped the team go undefeated in 2004, when he was named a first-team All-American.

During the 2005 NFL draft, Williams was selected by the Tampa Bay Buccaneers in the first round and also played for the St. Louis Rams.

He got the nickname "Cadillac" from a TV sportscaster[2] in Alabama for the style and the way he ran – he was a notch above everyone on the field, and the name stuck.

[6]) Williams was injured early in the bitter in-state rivalry game against the University of Alabama (also known as the Iron Bowl) with a broken collar bone and had to sit out the rest of the season.

Brown had completed a stellar season the previous year after picking up the starting role when Williams went down with a broken ankle.

However, Brown pulled a hamstring early on in the year and sat out most of the rest of the season, leaving Williams as the sole starting running back.

Williams went on to have a sensational season setting an Auburn single-season record with 17 touchdowns, and gaining a career-high 1,307 yards on 241 attempts (5.4 avg.).

This turned out to be a wise decision as he, along with quarterback Jason Campbell, Brown and others, helped lead the Tigers to an SEC Championship and Sugar Bowl winning season, finishing with a perfect 13–0 record and a controversial #2 ranking behind the University of Southern California.

Williams went on to earn All-American honors and was named to the All-SEC first-team as both a running back and return specialist.

Jeff Reynolds of Pro Football Weekly predicted Williams was the rookie most likely to have the greatest impact in the 2005 NFL season, writing that "at the Senior Bowl, Jon Gruden was salivating over this kid.

Tight end Heath Miller of Pittsburgh and offensive lineman Logan Mankins of New England also received one vote.

Williams was unable to match the success of his rookie campaign in the 2006 NFL season, as he failed to reach the 1,000 yard mark.

In October 2007, Williams suffered a torn patellar tendon to his right knee against Carolina, which required surgery and ended his season.

One month later, however, during the final game of the season on December 28, Williams suffered another torn patellar tendon, this time in his left knee.

The second tear was not as severe as the first, but Williams again endured surgery and an intense period of rehab to get back on the field.

[18] On December 27, Williams had his first 100-yard rushing game in two seasons for the Tampa Bay Buccaneers and finished with 823 yards, the second highest total of his NFL career.. Williams was narrowly edged for the AP 2009 Comeback Player of the Year Award by Tom Brady, finishing second with 15 votes.

However, due to the emergence of undrafted rookie sensation LeGarrette Blount, Williams found himself cemented as primarily the third down running back for the latter half of the 2010 Tampa Bay Buccaneers season.

Williams was used as a passing threat on third down and finished the season with 46 receptions for 355 yards and one touchdown (a come-from-behind, game-winning catch from Josh Freeman with ten seconds left in the fourth quarter on October 24, 2010, against the St. Louis Rams).

He finished the 2010 NFL season with 125 rushing attempts for 437 yards and two touchdowns (one against the San Francisco 49ers and one against the Carolina Panthers), helping his team to a 10–6 record.