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The 25-year-old outside half had spent his entire career in Llanelli, having come through the old academy system.
He said: "I thank the Scarlets for all the encouragement they've given me during my time here. I have some great memories from my time with my team mates. I have enjoyed meeting many of the great Scarlets supporters and I'm sure the region will go from strength to strength.
"However, I felt it was time for a new challenge and this is a great time to be going to Leeds Carnegie. They have a wonderful set-up and I'm relishing the prospect of playing in the Guinness Premiership."
Hereford-born Thomas made his debut for the Scarlets in the 2003-04 season. He went on to taste action in the Heineken Cup, Magners League and EDF Energy Cup.
The 25-year-old made 71 appearances for the Welsh region, scoring seven tries. His 288 career points haul included 62 conversions, 42 penalty kicks and a drop goal.
The 2008-09 campaign brought 19 Scarlets appearances including a game against the Barbarians which saw him bag 10 points.
Scarlets head coach Nigel Davies said: "We wish Ceiron all the best for the future. In a squad featuring the world-class fly-half talent of Stephen Jones he has had to be patient; he has always been an excellent professional."
