Molar mass dispersity in polymers affects a wide range of important material properties, yet there are few synthetic methods that systematically generate unimodal distributions with specifically tailored dispersities. Here, we describe a general method for tuning the dispersity of polymers synthesized via atom transfer radical polymerization (ATRP). Addition of varying amounts of phenylhydrazine (PH) to the ATRP of tert-butyl acrylate led to significant deviations in the reaction kinetics, yielding poly(tert-butyl acrylate) with dispersities D = 1.08-1.80. ATRP reactions in the presence of the reducing agent tin(ii) 2-ethylhexanoate, under otherwise comparable reaction conditions, did not drive similar increases in dispersity. We therefore deduced that PH does not function primarily as a reducing agent in these syntheses. Nuclear magnetic resonance analyses revealed the incorporation of aromatic polymer end-groups upon PH addition, suggesting that the ATRP-active halide termini of the growing polymer chains underwent irreversible nucleophilic substitution reactions with PH that led to chain termination. A kinetic model including this irreversible chain termination by PH was in excellent agreement with experimentally measured reaction kinetics. To demonstrate the generality of this approach, we conducted ATRP syntheses of polystyrene in the presence of PH to achieve dispersities of D = 1.07-2.30. This study suggests that PH addition is an effective, facile, and flexible method of dispersity control in polymers synthesized by ATRP.