The 6 ft 2 in (188 cm), 195 pound point guard grew up in New Jersey, and attended St. Joseph High School in Metuchen, graduating in 1999. He not only excelled at basketball, but took an active interest in other activities, most notably chess. His nickname in high school was "Jay Dubs." Williams also played junior varsity soccer during his freshman year and varsity volleyball during his senior year. In basketball that year, Williams was named a First Team All-State Player in New Jersey, the New Jersey Player of the Year, a Parade All-American, a USA Today first team All-American, and a McDonald's All-American, where he competed in the Slam Dunk Contest and the McDonald's All-American Game, scoring 20 points in the contest. He was also named the recipient of the 1999 Morgan Wootten Award for his basketball achievements and his work in the classroom, where he maintained a 3.6 GPA.
At Duke, Williams became one of the few freshmen in school history to average double figures in scoring and was named ACC Rookie of the Year and National Freshman of the Year by The Sporting News, averaging 14.5 points, 6.5 assists and 4.2 rebounds per contest. He was also a first team Freshman All-American by Basketball Times.
The next season Williams started all 39 games and led the Devils to the 2001 NCAA National Championship, earning NABC Player of the Year honors. His 841 points broke Dick Groat’s 49-year Duke record for points in a season, while he led all tournament scorers with a 25.7 ppg average. Williams also set the NCAA Tournament record for three-pointers attempted (66), while also making 132 three-point field goals—good for the sixth-highest total in NCAA history. His 21.6 ppg led the ACC and made him the first Duke player since Danny Ferry (1989) to lead the league in scoring. His 6.1 assists were good for second in the league, while he also ranked second in three-point field goal percentage (.427) and first in three-pointers made (3.4 per game). Williams was widely considered the best player in college basketball, earning both the prestigious Naismith Award and Wooden Award as College Basketball's Player of the Year in 2002. He graduated with a degree in Sociology in 2002, and left Duke with 2,079 points, good for sixth all-time, and with his jersey number 22 retired at Senior Day.
In 2001–02, Williams, Carlos Boozer, and Mike Dunleavy, Jr. each scored at least 600 points for the season, a feat only matched at Duke by Jon Scheyer, Kyle Singler, and Nolan Smith in the 2009–10 season. Williams (841) and Shane Battier (778) on the 2001 national championship team were one of only two Duke duos to each score over 700 points in a season, the other duo being Scheyer (728) and Singler (707) in the 2009–10 season.
On the night of June 19, 2003, Williams crashed his motorcycle into a streetlight at the intersection of Fletcher and Honore streets on Chicago's North Side. Williams was not wearing a helmet, was not licensed to drive a motorcycle in Illinois, and was also violating the terms of his Bulls contract by riding a motorcycle. Williams injuries included a severed main nerve in his leg, fractured pelvis and three dislocated ligaments in his left knee including the ACL. He required physical therapy to regain the use of his leg. A week after the motorcycle crash the Bulls drafted point guard Kirk Hinrich. When it became clear Williams would not be returning to the Bulls for a long time, if at all, because of his injuries, he was waived. The Bulls legally did not have to pay him any salary because his injuries occurred while he was violating his contract by riding a motorcycle. Instead the Bulls gave Williams $3 million when they waived him, which Williams could use toward future rehabilitation expenses. Williams stated that he would continue to train and make a return to the Bulls. In the interim, he appeared in college and high school basketball broadcasts on ESPN as a commentator.
Williams made an attempt to continue playing basketball. Although the Bulls no longer expressed much interest, on September 28, 2006, the New Jersey Nets announced that it had signed Williams to a non-guaranteed contract, giving Williams the opportunity to play in his home state. However, less than a month later on October 22, the Nets released Williams.
Williams subsequently announced that he has no plans to resume his basketball career. He did, however, complete a tryout with the Miami Heat prior to the 2010-11 NBA season, but was not picked up by the Heat.