Advancements in functional nanomaterials and wearable electronics have demonstrated the use of solid-state ion-selective electrodes (SS-ISEs) for human health applications. Existing devices, however, still rely on separate and multiple components of flexible sensors, interconnectors, and rigid data acquisition units, which limits the wearability of the entire system on the skin for continuous analyte monitoring. Here, this paper introduces an all-in-one, wireless, fully flexible, sodium detection system that integrates gold-carbon nanotube-gold (Au/CNT/Au) sensors and flexible thin-film circuits, together on a soft elastomeric membrane. The nanocomposite sensor includes electrochemically deposited Au nanoparticles on a CNT transducer to improve the conductivity, capacitance, and interfacial contact between materials. A set of experimental electrochemical analysis confirms the high stability of the SS-ISE based on Au/CNT/Au nanocomposites. At the same time, the thin-film system shows mechanical robustness and reliability, even with repetitive bending up to 500 cycles. The fully flexible, sensor-circuit integrated system demonstrates stable sodium measurements when mounted on the skin with minimized motion artifacts. The measured sensitivity of the sodium sensor is 55.5 ± 0.3 mV/decade, along with less than 3% change when the entire device is mounted on the skin with continuous movements. Overall, the presented comprehensive study, including nanomaterials, nano-microfabrication, electrochemistry, and electronics, shows the enormous potential of the wireless all-in-one sensor system for seamless integration with various types of skin-mounted wearable health monitors.