A key challenge in the synthesis of multicomponent nanoparticles (NPs) for therapy or diagnosis is obtaining reproducible monodisperse NPs with a minimum number of preparation steps. Here we report the use of microfluidic rapid mixing using hydrodynamic flow focusing in combination with passive mixing structures to realize the self-assembly of monodisperse lipid-polymer and lipid-quantum dot (QD) NPs in a single mixing step. These NPs are composed of a polymeric core for drug encapsulation or a QD core for imaging purposes, a hydrophilic polymeric shell, and a lipid monolayer at the interface of the core and the shell. In contrast to slow mixing of lipid and polymeric solutions, rapid mixing directly results in formation of homogeneous NPs with relatively narrow size distribution that obviates the need for subsequent thermal or mechanical agitation for homogenization. We identify rapid mixing conditions that result in formation of homogeneous NPs and show that self-assembly of polymeric core occurs independent of the lipid component, which only provides stability against aggregation over time and in the presence of high salt concentrations. Physicochemical properties of the NPs including size (35-180 nm) and zeta potential (-10 to +20 mV in PBS) are controlled by simply varying the composition and concentration of precursors. This method for preparation of hybrid NPs in a single mixing step may be useful for combinatorial synthesis of NPs with different properties for imaging and drug delivery applications.