Inspired by the brick-mortar structure, multi-functional polymer nanocomposites with carbon nanofillers were fabricated via an interface filler network design strategy, applicable for both thermoplastics and thermosets. This strategy enables constructing conductive networks at polymer-polymer interfaces in a facile and environment-friendly way by forming a segregated filler network at the polymer particle interfaces under appropriate temperature and pressure conditions. By comparison with melt mixing, this strategy shows advantages for the fabrication of elastic conductive composites, allowing potential applications for stretchable sensors. Moreover, by effectively controlling parameters like processing temperature, polymer particle size and filler dimensionality, tunable mechanical and electrical properties for different target applications can be obtained. For example, when using thermoplastic polyurethane (TPU), stretchable strain sensors and reversible pressure sensors with tunable piezoresistive sensitivity and sensing strain ranges were obtained. These can be used in body motion detection for monitoring respiration rates or guiding squatting in daily exercise. This strategy is also applicable for making recyclable thermoset composites with significantly improved mechanical properties at low filler loadings when using vitrimerized epoxy waste as the polymer matrix. These materials have good electrical conductivity, suitable for electromagnetic interference shielding and structural health monitoring applications.