A novel microelectromechanical systems (MEMS) process is developed to fabricate large numbers of high-performance MEMS devices monolithically integrated onto a rigid-flex organic substrate using low-temperature processes. The rigid-flex substrate is all dielectric, which is amenable to low-loss electromagnetic structures. The substrate provides mechanical support to the MEMS devices while maintaining overall flexibility. The newly developed process is used to fabricate a MEMS reconfigurable frequency-selective surface (FSS). A practical bias network is incorporated into the structure design to ensure that all devices are actuated simultaneously. A detailed parametric sensitivity analysis establishes the robustness of the FSS design with respect to fabrication process variations. FSS structures operating in the Ku- and Ka-bands are fabricated and tested with good correlation between simulated and measured results. The newly developed MEMS process is also used to fabricate a reconfigurable electromagnetic-bandgap (EBG) structure. An EBG structure operating in the Ka -band is fabricated and tested to verify the validity of the proposed concept.