Farrell, who played 112 times for England and had spent his entire professional career with Saracens before his summer move, only had to wait 10 minutes before notching his first points for his new club, converting a try by compatriot Henry Arundell.
Farrell converted another three tries, including one from Maxime Baudonne two minutes from time as Racing clawed back 12 points in the last 10 minutes and put themselves within reach of an impressive away victory.
However, two penalty misses from kicks he might normally nail, still left Racing vulnerable. A last minute piece of brilliance the home side's new Cameroonian wing Christian Ambadiang snatched the win for Castres.
Catch up on Castres' win here.
Another high profile import, former Scotland captain Stuart Hogg, also ended on the losing side when his Montpellier team lost 26-22 at home to Lyon.
Hogg did notch his first try for Montpellier but last season's top try-scorer Baptiste Couilloud also opened his account as Lyon managed to win on the road in round one - something they failed to do in 13 games last season.
Farrell's old England teammate Manu Tuilagi fractured his hand in pre-season so missed Bayonne's opener at home to Perpignan. It ended in another last minute victory as Bayonne won 21-19.
The Catalans led for most of the game thanks to the boot of Italian international Tomasso Allan but a 50-metre penalty from Joris Segonds won the game for the Basques.
Clermont enjoyed a comfortable win as they hammered visitors Pau 39-7 with former Australia lock Rob Simmons scoring the second of their five tries.
In Saturday's late game Bordeaux-Begles host Stade Francais in a repeat of last season's semi-final which UBB won 22-20.
Champions Toulouse travel to Brittany to take on newly-promoted Vannes on Sunday.
