The young Scarlets lead by 6pts to nil midway through the first half but conceeded the games only try close to half time and with Cardiff fly half Nick Macleod adding the conversion and a penalty it was the Arms Park youngsters who lead by 10pts to 6 at the interval.
Cardiff are virtually assured of the league title ahead of next weekends visit to Bridgend to complete the formalities of the title success and will now play Neath in the cup final. The game will be played at a neutral venue and the date is yet to be decided.
Llanelli started by far the brighter of the two sides but despite their flow of possession failed to convert their advantage into points other than two penalties from fly half Caerlon Thomas, while Cardiff attacked from deep to create their only clear cut scoring opportunity which wing Elgan Jones rounded off wide out.
Fira fly half Macleod converted superbly and his penalty on the stroke of half time gave the home side a lead at the interval.
The visitors responded well after the restart with scrum half Mike Phillips tackled short of the try line after a quickly taken penalty had taken play 50 metres up field and fullback Barry Davies created problems for the home side with a kick upfield before Cardiff regrouped. The home side moved play to within sight of the posts and Macleod calmly added his second penalty of the evening to give his side a crucial 7-point.
Despite the disruption through injury to their front row, the Stradey Park youngsters dominated the set pieces and looked threatening in attack as the crowd of over 500 waited in expectation of a visitors score.
However, Llanelli tried in vain to secure the vital score losing possession at crucial times and despite their endeavour and resolve, the visitors failed to break the final line of defense as Cardiff held onto victory and the chance for their unique double of titles.
Young referee Ben Breakspear is making rapid strides as a referee. In just over a year he has gone from officiating a college game to being a touchline official during last weekend's RBS 6 Nations clash between Wales and Ireland at Principality Stadium.
Grand Slams, Triple Crowns and British and Irish Lions tours - Sir Gareth Edwards did it all. But what are his favourite moments in what was an amazing career? WRU TV finds out - along with his favourite players of today in the third and final instalment of our series with the greatest rugby player ever.
JPR Williams, Gerald Davies and Mervyn Davies were among those in the Welsh ranks when Sir Gareth Edwards began his international career against France nearly 50 years ago, but who was the player who took the young scrum half under their wing?
Wales head coach Rob Howley and captain Alun Wyn Jones are expecting a physical battle against France on Saturday but both are hoping for another impact from the bench as the men in red chase a top-four world ranking.
It's nearly 50 years since the great Sir Gareth Edwards made his debut as a youngster for Wales against France in Stade Colombes. In the first of a series, WRU TV discovers he pocketed a 'special momento' from the game - something he still has today.
This week, Wales squad players Sam Warburton, Jonathan Davies and Ken Owens took time out to support Operation Net Safe, a joint initiative by the four Welsh police forces to tackle child sexual abuse. The forces are working with the Lucy Faithfull Foundation, the only UK-wide child protection charity dedicated solely to reducing the risk of children being sexually abused, on a campaign called Stop it Now!