In May 2019, the Phoenix Suns announced they had signed Williams as the team's head coach on a five-year deal. The Suns compiled a 26–39 record in his first season coaching them before the season was postponed due to the COVID-19 pandemic. The Suns were later invited to the 2020 NBA Bubble in order to play eight seeding games, where Williams coached the Suns to an 8–0 record, improving their overall record that season to 34–39. Despite this, the Suns failed to qualify for the play-in tournament to enter the 2020 NBA playoffs.