When a drop of a colloidal solution of nanoparticles dries on a surface, it leaves behind coffee-stain-like rings of material with lace-like patterns or clumps of particles in the interior(1-6). These non-uniform mass distributions are manifestations of far-from-equilibrium effects, such as fluid flows(1) and solvent fluctuations during late-stage drying(2). However, recently a strikingly different drying regime promising highly uniform, long-range-ordered nanocrystal monolayers has been found(7,8). Here we make direct, real-time and real-space observations of nanocrystal self-assembly to reveal the mechanism. We show how the morphology of drop-deposited nanoparticle films is controlled by evaporation kinetics and particle interactions with the liquid - air interface. In the presence of an attractive particle - interface interaction, rapid early-stage evaporation dynamically produces a two-dimensional solution of nanoparticles at the liquid - air interface, from which nanoparticle islands nucleate and grow. This self-assembly mechanism produces monolayers with exceptional long-range ordering that are compact over macroscopic areas, despite the far-from-equilibrium evaporation process. This new drop-drying regime is simple, robust and scalable, is insensitive to the substrate material and topography, and has a strong preference for forming monolayer films. As such, it stands out as an excellent candidate for the fabrication of technologically important ultra thin film materials for sensors, optical devices and magnetic storage media.