On 10 August 2016, Williams joined Everton on a three-year contract for a £12 million transfer fee.
He made his debut ten days later as a substitute in a 2–1 win against West Bromwich Albion at The Hawthorns, and three days after that, made his first start for Everton in a 4–0 EFL Cup win at home to Yeovil Town.
Williams scored his first Everton goal with an 86th-minute header to secure a 2–1 win over Arsenal on 13 December.
On 4 April 2017, he was sent-off at Manchester United for handling a goalbound shot by Luke Shaw, from which Zlatan Ibrahimović scored a penalty to equalise in a 1–1 draw.

Williams scored Everton's goal in their 2–1 UEFA Europa League group stage home loss to Lyon on 19 October 2017.
He was also booked in that match for pushing Lyon goalkeeper Anthony Lopes into an advertising board, prompting a brawl involving both sets of players and Everton supporters.
After the match, Everton manager Ronald Koeman said Williams needed to work on his self-control.
On 10 August 2016, Williams joined Everton on a three-year contract for a £12 million transfer fee. He made his debut ten days later as a substitute in a 2–1 win against West Bromwich Albion at The Hawthorns, and three days after that, made his first start for Everton in a 4–0 EFL Cup win at home to Yeovil Town. Williams scored his first Everton goal with an 86th-minute header to secure a 2–1 win over Arsenal on 13 December. On 4 April 2017, he was sent-off at Manchester United for handling a goalbound shot by Luke Shaw, from which Zlatan Ibrahimović scored a penalty to equalise in a 1–1 draw.
Williams scored Everton's goal in their 2–1 UEFA Europa League group stage home loss to Lyon on 19 October 2017. He was also booked in that match for pushing Lyon goalkeeper Anthony Lopes into an advertising board, prompting a brawl involving both sets of players and Everton supporters. After the match, Everton manager Ronald Koeman said Williams needed to work on his self-control.
Ashley Williams (footballer)