The development of high performance flexible electronics requires dielectrics possessing a high dielectric constant and enhanced mechanical stability. In this study, we report on novel polymer nanocomposite dielectrics based on n-octadecylphosphonic acid (ODPA) functionalized aluminum titanate (AT) nanoparticles as dopants in poly(4-vinylphenol) (PVP). The robust surface layer covering the nanoparticles induces improved dielectric properties of the resulting composite films due to better interfacial adhesion. The nanocomposite layer with tunable dielectric permittivity and low leakage current is highly compatible on flexible substrates and has been deposited via a simple solution process. The air-stable p-type and n-type transistors fabricated on the composite dielectric layer exhibited superior electrical performance than those on the pristine polymer dielectric layer. The complementary inverters made from these transistors possessed a large signal gain and sharp switching. We systematically studied the electrical performance of the capacitors, transistors and inverters under different applied strains on flexible substrates. From the flexibility test, all these devices were found to be mechanically stable and environmentally robust, demonstrating that the composite dielectric layer is an excellent candidate for the future development of conformable sensors, portable displays and other flexible electronic applications.