Fenby (26) joined the Scarlets in May 2009 following a spell with the Newcastle Falcons. From Rhos-on-Sea in Conwy, the former Llandudno youth player, is also a qualified chartered accountant and hasn't looked back since joining the Scarlets on a pro rugby contract and now goes into his fourth season with the West Wales region.
New Wales sensation, Williams (21) burst onto the regional rugby scene last season and his Scarlets success was quickly followed by time with the international set-up - winning his first Wales cap in June against the Barbarians at the Millennium Stadium.
Williams has appeared 26 times for the Scarlets with 40 points - scoring eight tries last season including a spectacular Heineken Cup try against Northampton Saints during the Scarlets memorable victory over the big hitting English club at Franklins Gardens last November.
Williams said: "It's been an unbelievable 12 months for me and to get my Wales cap was a real highlight following a run of games at the Scarlets last season against some of the top sides in Europe.
"It's been a quick learning curve but the support and development I've been given as one of the younger crop of players at the Scarlets has been excellent. They've given me some of the biggest opportunities you can get as regional player and I've just had the mindset to take every chance I get and put everything I have into each performance in a Scarlets shirt."
Fenby said: "It's great to be part of a squad that's getting stronger and more competitive and with the talent we have in our ranks now, particularly in the backs, it's an exciting place to be with a healthy level of competition and motivation for all the players to excel.
"At 26, I'm one of the older guys at the Scarlets now and I'm really enjoying my rugby and having a full pre-season this summer will be good for me. I was pleased with my form at the back end of last season - it was good to get game time and build my game after my injury and pull a string of decent performances together. I'm looking forward to next season, and pushing my game on further."
Scarlets Head Coach Simon Easterby 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."