Williams was on target three times in each half in just his second league start, with a 51-metre penalty in the first and a snap drop in the second the highlights with the boot.
But while Williams showed a maturity beyond his 20 years to pick up a well-deserved man of the match award, the Scarlets' 10th win of the RaboDirect PRO12 season was by no means a one-man show. The hosts were adventurous in attack and defiant in defence as they repeated their 13-6 success at Musgrave Park in Cork at the back end of November.
Munster never came close to threatening the Scarlets' line until wing Denis Hurley dotted down with the final play when even hope of a losing bonus point was well beyond the Irish giants.
And although Ireland centurion Ronan O'Gara uncharacteristically missed three shots at goal - including a sitter from straight in front of the posts and just 25 metres out - there was no doubting that the Scarlets were good value for a victory that sees them leapfrog the Ospreys into fourth spot in the standings.
Williams struck after 13 and 18 minutes to push the Scarlets 6-0 ahead and he was on target again a minute before the break after O'Gara had reduced the deficit to three points just before the half hour mark.
It was all Scarlets for the majority of the second period, with Williams striking twice more from the tee and then dropping his goal on 63 minutes when Munster skipper Mike Sherry was in the sin bin after the referee lost patience with persistent offending from the visitors.
With the game effectively dead and with Williams having seen yellow in the final minute, Hurley crossed after former Blues' centre Casey Laulala's clever step, with a heavily bandaged O'Gara adding the extras to ensure the scoreline didn't quite reflect the Scarlets' dominance.