In this article, we employ thin-film resonant waveguide grating structure on magneto-active elastomeric sponge using a delamination-assisted transfer process. Periodic nanostructured polymer layer was successfully separated from its host substrate and transferred onto a spongy elastomeric structure embedded with magnetic nanoparticles as a magneto-optical sensor. The fabricated magnetic sensor shows potential sensing enhancement from optical resonance characteristics. Based on a post-integration scheme, the presented approach can be applied to various nonplanar structures, which opens new avenues for lowcost novel sensing applications.heterogeneous integration, magnetic field sensor, magneto-active elastomer, photonic sensor, resonant waveguide grating