Creating porous carbonaceous membranes with durable mechanical properties and designed functionality is critical for the next generation of soft electronic devices; however, it has been proven extremely challenging. Herein, we report a facile strategy to fabricate highly porous carbon nanofibrous membranes with enhanced mechanical elasticity and intriguing functionality based on polybenzoxazine via combining multicomponent electrospinning and in situ polymerization. Tin oxide nanoclusters with diameters of 20–40 nm are homogenously distributed in the carbon matrix and on the surface of carbon nanofiber (CNF). A plausible plasticizing effect of the heterogeneous nanotextures endows the SnO₂/CNF membrane with robust mechanical elasticity and durability, which can maintain its original shape after serious deformation. Moreover, the elastic SnO₂/CNF membrane possesses a high surface area of 1415 m² g⁻¹ with a pore volume of 0.82 cm³ g⁻¹. With their integrated properties of extraordinary mechanical properties, high porosity, large surface area, and good electrochemical properties, the as-prepared SnO₂/CNF membranes exhibited a satisfactory capacitive performance with high energy density, ultralong cycling properties, and robust electrochemical stability against bending deformation, suggesting a promising usage as soft electrodes for flexible energy storage devices, and also opened up an avenue to the design of functional CNF materials with fine elasticity for various applications.