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.