A central challenge in the development of drug-encapsulated polymeric nanoparticles is the inability to control the nanoparticle physicochemical properties that affect their biodistribution, drug release, and efficacy. Nanoparticles may be developed by mixing and nanoprecipitation of polymers and drugs dissolved in organic solvents with non-solvents. Inadequate control over this mixing process is a source of variability in the synthesis of these nanoparticles by nanoprecipitation. We demonstrate that rapid and tunable mixing through hydrodynamic flow focusing in a microfluidic device can be used to control nanoprecipitation of poly(lactic-co-glycolic acid)-bpoly(ethylene glycol) (PLGA-PEG) diblock copolymers as a model polymeric biomaterial for drug delivery.