Mongongou began playing for his local club in La Garenne-Colombes before joining RCF Paris in 2002. He proceeded to join the Centre of Formation at Monaco in 2005 before making his first appearance in a 1-1 draw with Saint-Étienne on 4 August 2007. On 3 July 2009 he signed his first professional contract with Monaco. He went on to make a total of 92 appearances for the club.
In 2011, Mongongu joined Evian, making his debut on 21 September 2011 against Marseille.