In his junior year, he was kicked off the men's basketball team for several months, for attempting to sell stolen laptop computers along with teammate A. J.

In a game against Notre Dame, he recorded the sixth triple-double in UConn history with 18 points, 13 assists, and 10 rebounds.

[4] In the 2006 NCAA tourney, he averaged 20 points, 8.8 assists, while shooting 52% from the field, 56% from three-point range, and 96% from the free throw line.

[5] Williams was selected 22nd overall in the 2006 NBA draft by the New Jersey Nets, using the pick they got from the Denver Nuggets in a trade for Kenyon Martin.

On July 22, 2008, Marcus Williams was traded by the Nets to the Golden State Warriors for a conditional first-round pick.

In July 2009, Williams began playing on the Memphis Grizzlies Summer League team in Las Vegas, Nevada.

He joined 2009 first-round pick Hasheem Thabeet, undrafted rookie free agent Jeff Adrien, and Rudy Gay as one of four former UConn Huskies on the Grizzlies' Summer League roster.

[7] Williams signed with Piratas de Quebradillas of the Puerto Rican Basketball League (BSN) in late March 2009.

He also finished the season earning All-BSN First Team honors with teammate Peter John Ramos, and helping the Pirates to the best record in the league.

[9] Quickly becoming leader of his new team, Williams helped Enisey qualify to playoff for the first time in club's history.

[11] On April 26, 2018, Williams was reported to have signed with Piratas de Quebradillas of the Baloncesto Superior Nacional.

[28] On February 5, 2019, Williams was suspended for five games without pay for violating the terms of the league's anti-drug program.