Agen valiantly made them fight all the way, but Wales fly-half Stephen Jones’ ultimately saved the day.
The prolific Jones booted 14 points, which thwarted Agen’s victory hopes.
Yet there was nothing to suggest that Llanelli were good enough to land the European title.
Agen manufactured tries for wing Matthieu Dourthe and hooker Noel Curnier, while fly-half Jerome Miquel slotted a conversion and penalty.
Agen arrived in Wales without their French World Cup prop Jean-Jacques Crenca, who was sent off and banned for four weeks after he butted England hooker Steve Thompson during the Heineken Cup defeat against Northampton last month.
His fellow international, 35-year-old flanker Philippe Benetton, took over as captain but the squad knew they faced a huge task against Llanelli.
After posting pre-Christmas victories over Northampton and the Borders, Llanelli were determined to stay unbeaten.
Wales coach Steve Hansen was among a bumper crowd, running the rule over 10 Llanelli players named in his 47-man preliminary training squad for this season’s RBS Six Nations Championship.
Scorers: Llanelli Scarlets: Tries: S. Quinnell. Cons: S. Jones. Pens: S. Jones 4.
LLANELLI SCARLETS: B. Davies, Evans, Taylor, Watkins, Finau, S. Jones, Peel, I. Thomas, McBryde, J. Davies, Cooper, Wyatt, D. Jones, Easterby, S. Quinnell.
Scorers: Agen: Tries: Dourthe, Curnier. Cons: Miquel. Pens: Miquel.
AGEN: Elhorga, Lafforgue, Stoltz, Bonetti, Dourthe, Miquel, Tandonnet, Blanco, Curnier, Hasan, Kahla, Parent, Lievremont, Benetton, Sverzut.
Referee: D. McHugh (RFU).