Northampton started the game on top but it was Racing who grabbed the first try, the hosts' power freeing up space on the right for Henry Chavancy to cross.
© Getty Images
JJ Hanrahan kicked a penalty for Saints after 29 minutes to get their only points of the game, before Brice Dulin picked off an interception, with Racing under severe pressure on their own line, to go all the way.
Luther Burrell saw a try chalked off after he failed to properly ground the ball, before Dulin got his second three minutes after half time following strong work from his fellow backs in midfield.
The game broke down after this, with the weather doing its best to disrupt play, but Racing showed their class five minutes from time, turning the ball over and moving their way upfield before Juan Imhoff crossed with an easy finish.
Racing remain top of the pool with Saints sitting just behind them in second place, but with a game in hand.