Full-back Williams, recently capped by Wales against the Barbarians, has put pen to paper on a three-year contract with the west Wales region, while wing Fenby has agreed a 12-month extension.
Williams, 21, a former steelworks scaffolder, enjoyed a breakthrough campaign last season, scoring eight tries in 26 appearances, including a memorable effort in the Heineken Cup win at Northampton. Fenby, 26, has scored 11 tries in 50 appearances during his time at Parc y Scarlets.
And head coach Simon Easterby was delighted to see the pair commit their futures to the region.
He said: "It's great to secure one of the Scarlets' most talented young players on a long term contract - Liam has impressed everyone with his skills and commitment last year and we're sure he'll only get better.
"Andy and Liam are important players for us next season and it's important to secure the talent and commitment of these guys within our Scarlets squad to keep continuity and push forward our performances next season."