RNA-based therapeutics have revolutionized precision medicine due to their unprecedented potency, specificity, and adaptability. However, the inherent limited stability of RNA, including mRNA used in vaccines, is a major obstacle to the full realization of their potential. This instability, coupled with the centralized nature of vaccine production, currently limits the generation of RNA therapeutics at the point of care, which will otherwise fully harness the potential of these agents. Here, a microfluidic platform is presented for on-demand, personalized synthesis of modified mRNA stabilized by lipid nanoparticles. The design includes trapped biotinylated DNA, tagged T7 RNA polymerase, and a Tesla mixer, allowing the on-chip synthesis, purification, and encapsulation of mRNA in uniform lipid nanoparticles (LNPs), all conducted seamlessly on the same microfluidic device. This on-chip microfluidic synthesis approach is found to match standardized mRNA production yields, yet surpasses typical purification methods. Furthermore, as a proof-of-concept, the versatility and efficacy of the platform are demonstrated by generating diverse RNA sequences and structures, exhibiting functionality in human cell lines and mouse models. Moreover, an active SARS-CoV-2 vaccine is successfully engineered, highlighting the platform's potential for personalized vaccination strategies and offering a promising avenue for high throughput, decentralized vaccine delivery, reduced cold chain dependence, and even advancing current personalized medicine approaches through custom RNA therapeutics.