Magnetic core–shell structured catalysts, Fe₃O₄@Fe(II)–MOF nanoparticles (NPs), are synthesized in two steps including solvothermal synthesis of Fe₃O₄ NPs and self-assembly of Fe(II)–MOF particles on the core. They successfully enabled the real time and temporal control of an aqueous reversible addition–fragmentation chain transfer (RAFT) polymerization initiated by the Fenton reaction via an “on–off” reaction at room temperature. The Fenton reaction involves the reduction process of hydrogen peroxide (H₂O₂) by ferrous ions (Fe(II), inside the MOF), generating hydroxyl radicals (HO˙). These highly reactive radicals, therefore, initiate the magnetic Fenton-RAFT polymerization under ambient conditions. This process leads to the synthesis of controlled polymers with near quantitative monomer conversions, low dispersity values (Đ < 1.10), and experimental molecular weights in good agreement with the theoretical values. The versatility of this system in aqueous media is shown through the polymerization of acrylamide- and acrylate-type monomers including N-acryloylmorpholine, N,N-dimethylacrylamide, 2-hydroxyethyl acrylate, and poly(ethylene glycol) ethyl methyl acrylate, and the formation of polymers with different chain lengths. Moreover, the ‘living’ characteristics of the synthesized polymers are demonstrated with chain extension experiments and Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-ToF) analysis. It is expected that the concept of magnetic catalysis could facilitate such temporal control (or “on–off” reaction) over a wide range of polymerizations from a ring opening polymerization (ROP) system where cobalt (or tin)-based catalysts are needed to an atom transfer radical polymerization (ATRP) method where Fe (or Cu)-based catalysts are required.