The rational design and scalable assembly of nanoarchitectures are important to deliver highly uniform, functional films with high performance. However, fabrication of large-area and high-performance films is quite difficult because of the challenges in controlling homogeneous microstructures, interface properties, and the high cost of the conventional vacuum deposition technique. Here, we report a solution-processed molecular level assembly approach to fabricate self-supported (without any binders or conductive additives) large-area (up to 810 cm²) functional films with controllable thickness and high homogeneity. We show that the assembled prototypical Fe(II)-based metallo-supramolecular polymer (polyFe) film exhibits unprecedented electrochromic performance such as ultrahigh coloration efficiency (750.3 cm² C^–1), fast switching speed (<1 s), as well as robust electrochemical stability (with no obvious degradation after 10 000 cycles). We further demonstrate that the assembled polyFe films can be used to fabricate a smart energy-storage indicator, in which the energy-storage level is visually perceptible and recognizable in real time. This strategy provides an exciting alternative route for highly scalable fabrication of uniform films and may extend to other materials for a wide range of functional devices of diverse applications.