On 1 April 2017, Williams was a late replacement for the injured Conor Murray in Munster's line-up for the 2016–17 European Rugby Champions Cup quarter-final against Toulouse. Munster went on to win the game 41–16 and progress to the semi-finals. Williams earned his 150th cap for Munster on 10 February 2018, doing so when he started in the provinces' 33–5 win against Zebre in the 2017–18 Pro14. Facial injuries suffered during training initially ruled Williams out for the remainder of the 2017–18 season, as surgery was required, but he made a faster-than-expected recovery and returned to full training late in April. Williams was released by Munster at the end of the 2018–19 season.