Williams played college basketball at the University of Illinois where he led the Illini to the NCAA Championship game in 2005. He forgo his final year and entered the NBA Draft after 2005. Williams was drafted 3rd overall in the 2005 draft by the Utah Jazz, for whom he played before being traded in the middle of his sixth season to the Nets. He was a gold medal winner at the 2008 Summer Olympics.

On July 12, 2012, Williams signed a five year deal to remain with the Nets.