Tailoring the physical properties of hybrid lead metal halide APbX₃ perovskites by means of compositional engineering is one of the key factors contributing to the development of highly efficient and stable perovskite solar cells. While the beneficial effects of partial ionic replacement at the A- and X-sites are largely demonstrated, partial replacement of Pb²⁺ is less explored. Here, we developed a solution-based procedure to prepare thin films of mixed-metal MAPb_1-aMn_aI₃ perovskites. Although Mn²⁺ ions have a size that can potentially fit in the B-sites of MAPbI₃, using a combination of structural and chemical analysis, we show that only less than 10% of Pb²⁺ can be replaced by Mn²⁺. A 3% replacement of Pb²⁺ by Mn²⁺ leads to an elongation of the charge carrier lifetimes as concluded from time-resolved PL measurements. However, by analysis of the time-resolved microwave conductance data, we show that the charge carrier mobilities are largely unbalanced, which is in accordance with density functional theory (DFT) calculations indicating that the effective mass of the hole is much higher than that of the electron. Increasing the concentration of Mn²⁺ in the precursor solution above 10% results in formation of amorphous Mn-rich domains in the film, while the perovskite lattice becomes depleted of Mn²⁺. These domains negatively affect the charge carrier mobilities and shorten the lifetime of photogenerated carriers. The resulting reduction in charge carrier diffusion lengths will severely limit the photovoltaic properties of solar cells prepared from these mixed metal halide perovskites.