In this work, we report the relationship between the electrical conductivity and nanoparticle effective surface area with functional properties of polymer-metal and polymer-clay nanocomposites. Conductivity of the nanocomposite strongly depends upon metal/clay nanoparticle size and concentration that ultimately dictate where the system percolates. Knowledge of percolation properties allows the design of functional nanocomposites for biomedical and sensors applications. Herein we report the successful production of three functional chitosan-metal/clay nanocomposites: a) chitosan-Ag films with antibacterial properties, b) chitosan-Au potentiometric sensor for detection of Cu ++ and c) chitosan-nanoclay potentiometric sensor for detection of NO3-. For all these applications the best functional performance of nanocomposites has been observed when NPs concentration increases and approaches the percolation threshold. The obtained relationship between electrical percolation threshold and functional properties of polymer nanocomposites is of primary importance in the design of high-performance applications.