On 31 January 2006, during the winter transfer window, Mensah moved to French Ligue 1 club Rennes on loan after having played in Italy for most of his career, at the behest of their former player Philippe Redon, who brought him to fill the gap in defence caused by an injury to Abdeslam Ouaddou. Upon joining the club, he said: "In fact, I'm going into the unknown! I don't know anything about France! I just remember playing in 2001 against the French Espoirs during the U20 World Cup in Argentina. I remember that there was Cisse in front. But I did however have good advice on the level of the L1, in particular by my compatriot Mickaël Essien. I don't know many players in Rennes either. I just know that there is a good coach at the head of the team."