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The self-assembly of metal halide perovskite nanocrystals into micrometer-sized supercrystals with a high structural order as influenced by the surface chemistry and particle morphology of the starting building blocks is of interest for a broad spectrum of applications. In this work, we investigated the effects of Mn²⁺-doping CsPbBr₃ perovskite nanocrystals and their self-assembly into supercrystals. Mn²⁺ incorporation was found to improve the photoluminescence properties of both nanocrystals and supercrystals, resulting in higher photoluminescence quantum yields and longer radiative lifetimes compared to their undoped counterparts. Structural analysis using powder X-ray diffraction and electron microscopy confirmed that Mn²⁺-doping did not hinder the self-assembly of highly ordered, predominantly cubic supercrystals, but led to one-dimensional morphologies as dictated by the effect of increasing the Mn²⁺ molar ratio incorporated during nanocrystal synthesis. Notably, we observed a breakdown of three-dimensional supercrystal formation, driven by changes in the constituent nanocrystal size distribution controlled by Mn²⁺ addition, contrasting with previous studies where capping ligand density was the driving factor in these morphological changes. Furthermore, we showed through time-resolved powder X-ray diffraction and electron microscopy that the self-assembly of metal halide perovskite supercrystals occurs early in the slow solvent evaporation process, and superstructures can be formed on a variety of substrates, extending the range of application of these materials.