[3] Williams played at Snellville's South Gwinnett High School under Roger Fleetwood, where he was a four-time All-State selection and was named Georgia's "Mr. Basketball" as a junior and senior.

[6] Williams finished his career as the all-time leading scorer in Georgia high school basketball history.

After disappointing workout performances,[citation needed] Williams fell to the second round of the draft and was eventually picked 45th overall by the Philadelphia 76ers.

[7] After a good showing in the D-League,[8] and trade rumors surrounding 76ers' starting point guard Allen Iverson,[9] Williams was recalled back to the 76ers' roster on December 8, 2006.

At the end of that season, Williams became a restricted free agent and signed a five-year deal worth $25 million to remain with Philadelphia.

[10] In game 4 of the first round of the 2011 Eastern Conference Playoffs, Williams hit a go-ahead three-pointer to give the 76ers a win against the Miami Heat, 86–82.

[14] On January 18, 2013, against the Brooklyn Nets, Williams landed awkwardly on his right knee and had to leave the game on a wheelchair.

On June 30, 2014, Williams was traded, along with the draft rights to Lucas Nogueira, to the Toronto Raptors in exchange for John Salmons and a 2015 second round pick.

[16] On April 20, 2015, Williams was named the 2015 NBA Sixth Man of the Year, becoming the first member of the Toronto Raptors to ever win the award.

[19][20] He made his debut for the Lakers in the season opener against the Minnesota Timberwolves on October 28, 2015, scoring 21 points off the bench in a 112–111 loss.

[24] Two days later, he had 38 points, six rebounds and seven assists in a 107–101 loss to the Utah Jazz, putting together the first back-to-back 30-point games of his 12-year NBA career.

[26] On February 23, 2017, Williams was traded to the Houston Rockets in exchange for Corey Brewer and a 2017 first-round draft pick.

[29] On June 28, 2017, the Los Angeles Clippers acquired Williams, Patrick Beverley, Sam Dekker, Montrezl Harrell, Darrun Hilliard, DeAndre Liggins, Kyle Wiltjer, and a 2018 first-round pick from the Houston Rockets in exchange for Chris Paul.

[35] It was the highest scoring effort by any Clipper in a single game since Charles Smith tied a franchise record with 52 points in December 1990.

[42] On November 2, 2018, Williams scored 23 of his game-high 28 points in the second half of the Clippers' 120–95 win over the Orlando Magic.

[45] In his return to action on December 20, Williams scored 13 of his 26 points in the fourth quarter of the Clippers' 125–121 win over the Dallas Mavericks.

[46] On January 25, 2019, he recorded his first career triple-double with 31 points, 10 assists and 10 rebounds in a 106–101 win over the Chicago Bulls, joining Detlef Schrempf (in 1993) as the only other player in NBA history with a 30-point triple double off the bench.

[53] On March 17, he scored 25 points and hit a 28-foot 3-pointer from the top of the key at the buzzer to give the Clippers a 119–116 victory over the Brooklyn Nets.

[54] On April 15, 2019, Williams had 36 points and 11 assists to help the Clippers overcome a 31-point deficit—the biggest comeback in NBA playoff history—and tie the first-round series at 1–1 against the Warriors with a 135–131 win.

[60] In the 2020 NBA playoffs, he struggled throughout the postseason, having averages of 12.8 points and 4.2 assists as the Clippers fell in seven games against the Denver Nuggets while surrendering a 3–1 lead in the Western Conference semi-finals.

[61] On March 25, 2021, Williams, two second-round draft picks, and cash considerations were traded to the Atlanta Hawks in exchange for Rajon Rondo.

[67] He runs an annual summer camp at South Gwinnett High School, his alma mater, for children ages 10–16.

In December 2011, Williams claimed his NBA recognition saved him from an armed robbery attempt in the Philadelphia neighborhood of Manayunk.