A promising approach to fabricating elastic electronic systems involves processing thin film circuits directly on the elastic substrate by standard photolithography. Thin film devices are generally placed onto stiffer islands on the substrate surface to protect devices from excessive strain while still achieving a globally highly deformable system. Here we report a new method to achieve island architectures by locally reinforcing polymeric substrates at the macro- and microscale using magnetically responsive anisotropic microparticles. We demonstrate that the resulting particle-reinforced elastic substrates can be made smooth enough for the patterning and successful operation of thin film transistors with transfer characteristics comparable to state-of-the-art devices.