At the end of January 2011, he made his first appearance for the Scarlets, coming on as a replacement for Steven Shingler for the final seven minutes of a 34–7 Anglo-Welsh Cup defeat to Saracens.
[4] He started his first game two weeks later, playing at fly-half for Llanelli and scoring five points (one conversion and one penalty) in a 59–15 defeat to Aberavon.
Things got worse for Williams in October 2012, when he suffered a dislocated shoulder while playing for Llanelli in a 38–28 victory over Gala RFC in the British and Irish Cup, which kept him out for four months.
[1] After his recovery, he returned to the fly-half position for all eight of Llanelli's remaining Premiership games for which he was available, including the 47–15 loss to Pontypridd in the play-off final.
[30] Full-back duties were assigned to Steven Shingler, Josh Lewis and Kristian Phillips for the next three games, but Williams started the next five during the 2015 Six Nations Championship, scoring a try in the first of those in a 32–14 home win over Connacht on 15 February after making an interception on his own goal line,[31] and another in a 23–13 win at home to Leinster on 7 March.
[33] During the season, Williams also played three times for Carmarthen Quins in the British and Irish Cup,[34] scoring one try against Jersey Reds.
[37] He was far more active for Llanelli, playing nine times in the Welsh Premiership at wing, full-back and fly-half, and scoring seven tries and six conversions.
[40] He scored his first try for the club the following week, in a 41–17 home defeat to Exeter Chiefs; he also provided the conversion for Max Crumpton's late try.
[50] He scored again after coming off the bench in the team's very next match, at home to Clermont, but it proved to be only a consolation try, as the French side won 49–7 to eliminate the Dragons from the Challenge Cup.
[51] His final score of the season came on 4 April, when he came off the bench to help the Dragons come from behind to salvage an 18–18 draw away to the Southern Kings.
[58] Following the suspension and subsequent resumption of professional rugby union due to the COVID-19 pandemic,[59][60] Williams eventually made his comeback in the Dragons' European Challenge Cup quarter-final match against Bristol on 18 September 2020, almost 10 months after his injury, though he was unable to prevent his former side from winning comfortably, 56–17.
[61] Williams missed the first game of the 2020–21 season against Leinster,[62] but returned for the Dragons' 26–18 win at home to Zebre on 9 October, in which he scored two tries and set up Jamie Roberts for another.
[80] He was named on the bench for the final pool match against Argentina, and came on for Hallam Amos in the 59th minute as Wales won 25–20 to top the group and qualify for the semi-finals.
South Africa replied almost immediately, and had a six-point lead going into the final minute, when Williams broke four tackles to put Wales in good field position for Sam Davies to set up a try for wing Ashley Evans, which Davies then converted to give Wales an 18–17 win.
[86] In 2014, following an extended run in the Scarlets team at both wing and full-back to end the season, Williams was named in the Possibles squad for a Probables vs Possibles match ahead of Wales' summer tour to South Africa,[87] although the unavailability of some players meant he was picked on the wing for the Probables team for the match itself.