After returning home in an attempt to seal a regular place for France he rejoined the club in 2005, and has amassed 1,325 points and 15 tries in all.
He said: "Gloucester has been a really special place for me and I want to thank everyone involved with the club - particularly the supporters - for making my time here so enjoyable.
"It is right for my career that I move on at this time but I leave with many good memories."
Mercier is leaving to seek new challenges on the continent and head coach Dean Ryan added: "We wish Ludo every success in the future.
"His kicking game has served Gloucester well over the last 6 years and on behalf of everyone we thank Ludo for his commitment and service to the club."