Born in Salisbury, North Carolina, Williamson attended Spartanburg Day School, where he was a consensus five-star recruit and was ranked among the top five players in the 2018 class. He led his team to three straight state championships and earned South Carolina Mr. Basketball recognition in his senior season. Williamson also left high school as a McDonald's All-American, runner-up for Mr. Basketball USA, and USA Today All-USA first team honoree. In high school, he drew national attention for his slam dunks.