The theory of functionalism dominates the law of international organizations, explaining why organizations have the powers they possess, why they can claim privileges and immunities, and often how they are designed as well. Yet, the theory of functionalism is rarely spelt out in any detail, and its origins have remained under-explored. The purpose of the present article is to outline how functionalism came about by focusing on the "pre-history" of international institutional law. To that end, the article studies the work of a number of late 19th, early 20th century authors on the law of international organizations, paying particular attention to the writings of Paul Reinsch. It turns out that functionalism, as developed by Reinsch, was inspired by his familiarity with colonial administration: colonialism and international organization both manifested cooperation between states. While this is no reason to discard functionalism, it does provide an argument for viewing international organizations more critically than functionalism habitually does.