In 1872, Maxwell proposed his famous demon gedanken experiment. By discerning which particles in a gas are hot and which are cold, and then performing a series of reversible actions, Maxwell’s demon could rearrange the particles into a manifestly lower entropy state. The apparent violation of the second law of thermodynamics was resolved in the twentieth century: Maxwell’s demon often increases the entropy of the universe while gathering his information, and there is an unavoidable entropy increase associated with the demon’s memory. Despite its theoretical resolution, the appeal of the demon construct has led many experiments to be framed as demon-like. However, past experiments have either had no intermediate information storage, negligible change in the system entropy, or have involved systems of four or fewer particles. Here, we present an experiment that realizes the full essence of Maxwell’s demon. We start with a randomly half-filled 3D optical lattice with ~60 atoms. We can make the atoms sufficiently vibrationally cold that the initial disorder is the dominant entropy. After determining where the atoms are, we execute a series of reversible operations to create a fully filled sub-lattice, a manifestly low entropy state. Our Maxwell demon lowers the total entropy by a factor of 2.44, enough to cool the ensemble below the quantum degeneracy threshold. We plan to use this Maxwell demon to initialize a neutral atom quantum computer.