Williams made his Munster debut against Connacht in December 2009.[1] He made his Heineken Cup debut against London Irish in October 2010.[2] Williams also started against Australia in Munster's historic 15–6 victory over them in November 2010.[3] He scored his first try for Munster in their 35–12 win against Scarlets in September 2011.[4] He signed a two-year contract extension with Munster in March 2012.[5]
He started for Munster A in their 31–12 2011–12 British and Irish Cup Final win against Cross Keys on 27 April 2012.[6] On 28 February 2014, it was announced that Williams had signed a new one-year contract with Munster, which would see him remain in the province until at least June 2015.[7] He signed a two-year contract extension in February 2015.[8] On 2 January 2016, Williams earned his 100th cap for Munster when he came off the bench against Ulster.[9] On 24 January 2017, it was announced that Williams had signed a contract extension which would see him remain with Munster until June 2019.[10]
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.[11] 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.[12] Facial injuries suffered during training initially ruled Williams out for the remainder of the 2017–18 season, as surgery was required,[13] but he made a faster-than-expected recovery and returned to full training late in April.[14] Williams was released by Munster at the end of the 2018–19 season.[15]
After his release by Munster, Williams continued playing rugby at an amateur level for Cork Constitution. He had decided to retire from playing rugby after Con's All-Ireland League Division 1A final against Clontarf in May 2019,[16] but after Con won the final 28–13 to secure their second AIL title in three years,[17] Williams extended his playing career into the 2019–20 season, and he started in the 24–17 win against Young Munster in the final of the Munster Senior Cup in December 2019, a victory that secured a 30th title for the Cork side.[18]