<p>Lipid nanoparticles (LNPs) are nanosized vehicles for gene delivery that enabled great success of mRNA vaccines in past years. Much of the success in the LNP-related gene therapeutics—vaccination, immunotherapy, and others—is in debt to its ability in packaging nucleic acids of long sequence inside the core and in delivering cargos efficiently across the plasma and endosomal membranes. Nevertheless, LNPs are structurally far from equilibrium; the particle size as well as the mass of encapsulated genes may vary widely particle-wise without timely controlled and spatially homogeneous particle growth. This raises critical needs to ensure homogeneous state of mixing during the nucleation and growth stage of LNPs with timescale of ∼10 ms. This review addresses fundamental aspects in and recent success of toroidal micromixers for the production of well-defined therapeutic LNPs, especially in the context of gene delivery systems. Toroidal micromixer is a 2D passive microfluidic device that is simple to operate, inexpensive, and has a mixing time that is sufficiently short for homogeneous LNP growth. It uses a series of toroidal channels arranged in a zig-zag configuration to interweave Dean vortex and asymmetric split-and-recombine effects. The existing commercialized toroidal mixers significantly lowered the technical barrier of microfluidic LNP production for newcoming researchers, and design of advanced toroidal mixers is actively studied in the field. We discuss the effectiveness of toroidal mixers in the production of LNPs that meet the requirements in terms of the structure and bioactivity and major factors in the toroidal mixing devices that need to be tailored when using different formulation of lipids and genes. Lastly, we summarize recent studies that used toroidal mixers with various production scales for LNPs encapsulating genes and other therapeutic cargos.</p> Graphical abstract <p></p>