Marseille worked their way into the game and Benoit Cheyrou could have broken the deadlock twice, but he missed the target on both occasions.
Belghazouani continued to threaten for the hosts and he had another effort on 25 minutes but was unable to test Mandanda this time.
Marseille continued to threaten after the break and Ayew hit the opener on 55 minutes after being set up well by Charles Kabore.
Marseille's defence was playing a high line and Belghazouan and Adrian Mutu were caught offside as the visitors looked to hold on to their lead.
As Ajaccio looked for an equaliser Marseille wrapped up the points at the other end, Mathieu Valbuena setting up Ayew for 2-0.