Mensah made his Stade Rennais debut, starting the whole game, in a 2–0 loss against Bordeaux on 11 February 2006. Since making his debut, he quickly established himself for the club, playing in the centre–back position. Mensah then helped the side kept four consecutive clean sheets between 11 March 2006 and 31 March 2006. During in which, he scored his first goal for the club, in a 2–0 win against Troyes on 25 March 2006. Despite suffering from a thigh injury, Mensah made fourteen appearances and scoring once in all competitions. High-level performances in the league convinced Rennes to make the loan deal permanent in April 2006, which Mensah signed a three-year contract with the side.