As a senior, Williams was named an all-district and all-state player as he averaged 35 points, 14 rebounds, four blocks, three steals, and two assists per contest. During that time, the Crusaders were ranked in the state as high as number one, and was only defeated in the semi-final round of the state championships.
Williams completed high school in 2000. During his four years, Martin Luther King had a 68–16 win-loss record.
2000–2001: Williams scored 18 points in Oakland's upset of Michigan on November 17, 2000. As a rookie, he ranked among the team leaders in scoring (6th), rebounding (1st), assists (4th) and steals (3rd).
2001–2002: Williams played in every game for the Golden Grizzlies as a sophomore.
2002–2003: Williams played in every game for a third straight year for OU in 2002–2003, finishing third on the squad in rebounding.
2003–2004: Williams had a solid senior season, ranking among the team leaders in scoring (4th), rebounding (3rd), assists (4th) and steals (4th).
In 2004, Williams was spotted by then Philippine National Team head coach Chot Reyes after the latter was amazed with the former's work ethic and stellar play. Reyes invited Williams to try out for the Philippine Team and the high-flying forward did not hesitate, ensuing his arrival at the local basketball scene.
After displaying superb skills in exhibition games with the Philippine Team, he caught the attention of Filipino basketball aficionados, awarding him the title as the next big thing in Philippine basketball. Hence, he did not disappoint as he averaged 17.1 points and 12.8 rebounds during his stint with the Philippine Basketball League's (PBL) Magnolia.
When the Realtors took the first overall draft right in 2006, they did not hesitate to pick Kelly Williams over two-time UAAP MVP Arwind Santos, who was later picked second overall by the Air21 Express. During his rookie season, he normed averages of 17.3 points and 9.6 rebounds earning him the Rookie of the Year award, making him the second Realtor to do so after Jun Limpot in 1993. In his sophomore year, he improved his game immensely, helping his team win their second championship overall and grabbing the league's Most Valuable Player award along the way.
In 2009, during the All-Star Week, Williams was diagnosed of thrombocytopenia and was out for the remaining games of the entire conference. With his absence, his team did not last long and was eliminated by the Burger King Whoopers. He was also cut from the training pool for the national team because of his illness.
Kelly Williams was out indefinitely for the rest of the 2013-13 season due to a recurring illness.
The 2008 PBA Most Valuable Player is suffering from idiopathic thrombocytopenic purpura, the same illness that caused him to miss a significant chunk of the 2008-09 season when he was still part of the Sta. Lucia Realtors.
He has since returned from his indefinite leave after he discovered an alternative medication.