Williamson attended Friendswood High School in Friendswood, Texas. In his senior year he posted a 0.68 ERA and was named district MVP. In college, Williamson played for first Tulane University and then Oklahoma State University. With the latter, he earned Big 12 first-team honors during the 1996-1997 school year. Williamson entered the draft after the season was over and was selected by the Cincinnati Reds in the 9th round, 276th pick overall.