WORLD'S LARGEST MONKEY. THE LONGEST LIST OF THE LONGEST STUFF AT THE LONGEST DOMAIN NAME AT LONG LAST. What is the world's largest monkey? The Mandrill monkey which is related to the baboon has the distinction of being the world's largest monkey as far as species go. The Mandrills occupy the tropical rainforests of West Africa where they spend their days foraging for plants, insects and smaller animals in groups as large as 200.