He is the leader of the soldier ants who envisions a colony as being inhabited by the "strong elements", and seeks to fulfill the vision by sending the soldiers on a suicide mission and having the workers dig a tunnel to their own deaths. He also wishes to marry Princess Bala and make her his queen as well as the mother of thousands of larval children, but she ultimately calls off the wedding and rejects him, having become suspicious of his plans. He is killed by falling to his death on a root at the near-end of the film.
Mandible is a nasty dictator of the ant colony. He even tricked them into going into war. He has strong feelings/lust for Bala but Bala doesn't like him. Mandible later interrogates Weaver into wanting to know where Z is located. He is described as crazy by Bala later on in the film.
General Mandible was the former leader of the soldiers and he was allowed by the Queen to make decisions of the colony.