Efficient synthesis of polymeric microspheres with high uniformity, well-defined surface functionality, and precise diameter and composition has long been a challenging goal in polymer science. Herein, we exploited photoinitiated reversible addition-fragmentation transfer (RAFT) dispersion polymerization of methyl methacrylate using poly(glycerol monomethacrylate)-based macromolecular chain transfer agents (macro-CTAs). We showed that the use of a binary mixture of macro-CTA and CTA with poor controllability was crucial for obtaining monodisperse poly(methyl methacrylate) (PMMA) microspheres. Evolution of PMMA microspheres during the polymerization was followed by scanning electron microscopy and H-1 NMR spectroscopy, confirming a linear evolution of particle volume with monomer conversion. PMMA microspheres with better colloidal stability were obtained when increasing the amount of macro-CTA used in photoinitiated RAFT dispersion polymerization. Finally, a two-stage photoinitiated RAFT dispersion polymerization was also explored by adding additional monomers in the second stage. We demonstrated that the particle size of polymeric microspheres could be precisely controlled by adding different amounts of the monomer in the second stage. Moreover, polymeric microspheres composed of two polymers were also prepared by adding a different monomer in the second stage. This study not only optimizes reaction conditions for preparing monodisperse surface-functional polymeric microspheres but also provides mechanistic insights into photoinitiated RAFT dispersion polymerization.