Williams completed high school in 2000. During his four years, Martin Luther King had a 68–16 win-loss record.

In 2004, Williams was spotted by then Philippine National Team head coach Chot Reyes after the latter liked the former's work ethic and style of play. Reyes invited Williams to try out for the Philippine Team. Williams agreed to it, ensuing his arrival at the local basketball scene.

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 2008, he was again named into the RP Training Pool under Coach Yeng Guiao. He was also entitled the PBA Most Valuable Player the same year.

Williams suffers from idiopathic thrombocytopenic purpura. He was first diagnosed with the recurring rare blood disorder in 2009. Since then, he has been taking regular medications.

In the middle of 2010 PBA Fiesta Conference, Williams, along with teammates Ryan Reyes and Charles Walters, was traded to the Talk N Text Phone Pals for Ali Peek, Nic Belasco, Yousif Aljamal, and Ogie Menor. The deal was brought about by Sta. Lucia's financial problems. Since joining Tropang Texters, in 2010, he won five championships.

In 2011, Williams suited up and was the starting power forward for the Smart Gilas national basketball team, the national team of the Philippines who finished 4th in the 2011 FIBA Asia Championship.