Various forms of wearable electronics have been developed, including wrist bands, smart glasses, and watches, which are exposed extensively to outdoor activities. The key metrics for these rapidly-emerging, wearable electronics are high touch sensitivity and good mechanical and thermal stability of the flexible touchscreen panels (TSPs). The dielectric constants (k) of their protective cover layers are very important because they can determine touch sensitivities. Thus, studies on cover layers that are flexible and transparent and that have high-k dielectric films with outstanding mechanical and thermal reliabilities are essential for the success of future wearable electronics. Herein, we report an unconventional approach for forming flexible and transparent cellulose nanofiber (CNF) films. These films are used to embed random networks of ultra-long metal nanofibers that serve as nanofillers to increase the value of k significantly (above 9.2 with the high transmittance of 90%). Also, by controlling the dimensions and aspect ratios of these metallic fillers, we studied the effects of the fillers’ nanostructures and contents on the optical and dielectric properties of the resulting films. The length of the metal nanofibers can be controlled precisely using a stretching method locally to break the highly-aligned, ultra-long nanofibers. These nanofiber-embedded, high-k films are mechanically and thermally stable, and they have a better Young’s modulus and better tensile strength with lower thermal expansion than commercial transparent plastics. Our demonstration of highly-sensitive, flexible TSPs using high-k CNF film for smartphones suggested that this film has significant potential for use in next-generation, portable electronic devices.