Williamson was born Thomas S. Williamson III in Washington, D.C., the son of Tom Williamson and Shelley Brazier. He is one of four children. His father is a senior attorney at Covington & Burling, a prestigious law firm in D.C. He attended Gonzaga College High School where he played basketball, track and field and participated in the musical theatre program. Williamson graduated in 2008. He attended the University of Southern California and graduated in 2012 with a degree in communications and media studies.