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The extreme sensitivity of the metal–insulator (M–I) transition in RNiO3(R = rare-earth ion) nickelates to various extrinsic and intrinsic factors rely on mechanisms driving structure–property relations. Here, we demonstrate a unique way to control the M–I transition of epitaxial Pr0.5Sm0.5NiO3thin films using a mosaic template of the LaAlO3(100) substrate; two sets of epitaxial films were deposited on highly oriented crystals and mosaic (with multiple crystallites) crystals. While the former films exhibit a robust and sharp M–I transition, the films on the mosaic substrate show distinctively much more subtle and broad transition, albeit same factors suggesting compositional purity. Terahertz (THz) dynamic conductivity too behaves very differently for the two types of films; Drude dynamics dominate the conductivity of highly crystalline films, whereas disorder-driven Drude–Smith conductivity prevails in mosaic films. Using this mosaic structure-controlled M–I transition and conductivity dynamics, we propose to implement these two templates of films for digital and analog THz transmission amplitude modulators.