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Abstract Polymer‐brush‐grafted nanoparticles (PGNPs) that can be covalently crosslinked post‐processing enable the fabrication of mechanically robust and chemically stable polymer nanocomposites with high inorganic filler content. Modifying PGNP brushes to append UV‐activated crosslinkers along the polymer chains would permit a modular crosslinking strategy applicable to a diverse range of nanocomposite compositions. Further, light‐activated crosslinking reactions enable spatial control of crosslink density to program intentionally inhomogeneous mechanical responses. Here, a method of synthesizing composites using UV‐crosslinkable brush‐coated nanoparticles (referred to as UV‐XNPs) is introduced that can be applied to various monomer compositions by incorporating photoinitiators into the polymer brushes. UV crosslinking of processed UV‐XNP structures can increase their tensile modulus up to 15‐fold without any noticeable alteration to their appearance or shape. By using photomasks to alter UV intensity across a sample, intentionally designed inhomogeneities in crosslink density result in predetermined anisotropic shape changes under strain. This unique capability of UV‐XNP materials is applied to stiffness‐patterned flexible electronic substrates that prevent the delamination of rigid components under deformation. The potential of UV‐XNPs as functional, soft device components is further demonstrated by wearable devices that can be modified post‐fabrication to customize their performance, permitting the ability to add functionality to existing device architectures.