Post-polymerization modification (PPM) has been broadly employed to achieve functional polymer brush surfaces via immobilization of functional moieties on the brush using efficient organic tranformations. Here, we demonstrate the amine-anhydride reaction as a modular PPM route to functional brush surfaces using poly(styrene–maleic anhydride) (pSMA) copolymer brushes as a platform. The amine-anhydride reaction on pSMA surfaces proceeds to high conversions, with rapid kinetics, under ambient reaction conditions, and exploits a readily available library of functional amines. Using cystamine as a modifier, a convenient route to thiol-functionalized brushes was developed that enables sequential PPM modifications with a large library of alkenes using both base-catalyzed thiol-Michael and radical-mediated thiol–ene reactions. The high fidelity PPM reactions were demonstrated via the development of multifunctional, micropatterned brush surfaces.