Advanced hydrogel interfaces exhibiting finely tuned mechanical characteristics and diffusion properties are essential in wearable and implantable biosensors, addressing tissue–device mismatches and controlling target analyte transport in biofluids. This work presents an ion-mediated structural engineering approach designed to meticulously regulate the porous architecture and mechanical robustness of poly(vinyl alcohol)-alginate hydrogels (PAH) through straightforward ionic modulation, effectively addressing inherent tradeoffs between mechanical strength and analyte diffusion. Utilizing three complementary ionic mechanisms—salting-out, calcium ion chelation, and sequence-directed biomineralization—hydrogels with tailored porous microstructures are fabricated. The resulting hydrogels exhibit pore sizes ranging from 65 nm to 2.5 µm, mechanical moduli of 50 to 140 kPa, and controlled analyte diffusion behaviors. Leveraging this structural tunability, two exemplary glucose biosensors are demonstrated: a highly porous hydrogel-integrated wearable biosensor designed for rapid and sensitive glucose monitoring in sweat, and a densely structured hydrogel-integrated implantable biosensor optimized for robust and continuous glucose tracking in interstitial fluid. This innovative methodology elucidates critical interconnections between the hydrogel’s ion-mediated microstructural architecture, its mechanical robustness and tunable diffusion characteristics, and the resulting biosensing performance optimized for wearable and implantable applications, thereby advancing the design paradigm for next-generation personalized biosensor interfaces.