An iron porphyrin catalyst, derived from the active center of proteins such as horseradish peroxidase and hemoglobin, was successfully used for the atom transfer radical polymerizations (ATRP) of methacrylic acid. ATRP of methacrylic acid and other acidic monomers is challenging due to Cu complexation by carboxylates, protonation of the ligand, and displacement of the halogen chain end. A robust mesohemin-based catalyst provided controlled ATRP of methacrylic acid, yielding poly(methacrylic acid) with Mn ≥ 20000 and dispersity Đ < 1.5. Retention of halogen chain end was confirmed by successful chain extension of a poly-(methacrylic acid)-Br macroinitiator.