For neat Pb perovskites, two-dimensional (2D) hybrid perovskites, where n layers of inorganic material are separated by a long-chain organic cation, generally exhibit greater stability but have lower photovoltaic performance characteristics, motivating the study of 2D/3D mixed-dimension systems to realize both high efficiency and stability. In this Letter, we demonstrate such optimal compromise between performance and stability using formamidinium, cesium, and t-butylammonium as A-site cations with Pb:Sn mixed-metal low-band-gap perovskites. Perovskite solar cells based on n = 4 and 5 lead–tin perovskites achieved power conversion efficiencies of up to 9.3 and 10.6%, respectively, and correspondingly retained 47 and 29% of their initial efficiency during storage in nitrogen for 2000 h. A similar stability trend for n = 4 over n = 5 was also observed for unencapsulated devices during continuous operation under a combined air atmosphere and temperature for 10 h, resulting in improved stability over the 3D lead–tin counterpart.