Elderly and an avowed pacifist, Mandemus appears to bear no grudges toward his former human captors, and wants everyone to simply live in peace. The nuclear war that destroyed much of the planet (including the city which the apes narrowly escaped) proved the futility of weapons and fighting to Mandemus. With such an attitude, the ape leader Caesar appointed him keeper of Ape City's small armory, "and of Caesar's conscience", reasoning that Mandemus would talk him out of anything impulsive or unwise.
Besides his other duties, Mandemus also became a teacher in Ape City's first school. One of his students was the genius orangutan Virgil, who also became a teacher, and an adviser of Caesar.