To maintain the momentum and impact of the field, assembled materials systems must increasingly incorporate broad functionality to meet real-world applications. Here we describe nanocomposite films of specially synthesized inorganic Prussian blue (PB) nanoparticles and linear poly(ethylene imine) (LPEI) that possess the unusual functional combination of high-performance electrochromism for displays and controllable dissolution for drug delivery. Fabrication using layer-by-layer (LBL) assembly was followed by spectroelectrochemical characterization, allowing a full composition determination rarely achieved for LBL films. The electrochromic performance of thick LPEI/PB nanocomposites most relevant to applications surpassed that of inorganic PB films with competitive switching speed and superior contrast. Oxidation beyond the primary electrochromic transition removes nanoparticle ionization and can controllably dissolve the films. Because PB is non-toxic we suggest this mechanism for controlled in-vivo drug delivery. The performance and multifunctional quality of these nanocomposites promise a strong impact on flexible displays, electrochromic windows, and even biomedical devices.