Williams attended Ridge Community High School in Davenport, Florida, where he was a three-sport star in football, track and basketball.

He won the 200-meter dash event at the 2010 East County Track Meet with a time of 22.49 seconds.

[6] During the 2014 BCS National Championship Game against Auburn he rushed for 25 yards on five carries, including a seven-yard fake punt conversion on fourth-and-four.

[20] In Buffalo's Week 10 win over the New York Jets, Williams tied a record held by former New England Patriots running back Robert Edwards by reaching the end zone in his sixth consecutive game to start his career.

[21][22] On August 20, 2016, Williams was released by the Bills after reportedly showing up to training camp out of shape and was later suspended four games for violating the NFL's substance abuse policy.

[27] On June 28, 2017, Williams was suspended for at least one year for violating the NFL Policy and Program for Substances of Abuse for a third time.

[29] Williams signed a futures contract with the Toronto Argonauts of the Canadian Football League on November 15, 2019.

Williams has seven children: two daughters, Kylie and Kota, and five sons, Karlos Jr., Kobi, Kason, Korri, and Karter.