The optoelectronic characteristics of organic–inorganic perovskites can be tailored by mixing different metal cations with various ratios. 2D layered organic–inorganic perovskites with charge transport anisotropy are considered as promising channel materials for field‐effect transistors. However, there are few reports about 2D Pb‐based perovskite thin film transistors although their Sn counterparts have been extensively investigated for use in this field. Herein, the synthesis and tuning of the physical properties of 2D layered Sn–Pb perovskite solid solutions based on phenylethylammonium tin and lead iodide perovskites ((PEA)2SnxPb1−xI4 (x = 0, 0.3, 0.5, 0.7, 1)) are reported. A thermally activated semiconducting behavior is confirmed in this series of perovskite thin films. The electronic band structure, conductivity, and in‐plane ion migration are found to be largely affected by the Sn–Pb ratio, which is important for transistor performance. Furthermore, the 2D layered mixed Sn–Pb perovskite field‐effect transistors constructed on cheap and commercially available polymer dielectrics and a signature of field‐effect characteristics in the (PEA)2PbI4 film are demonstrated for the first time. The (PEA)2Sn0.7Pb0.3I4 transistor operating at room temperature in air exhibits a hole mobility of 0.02 cm2 V−1 s−1. This work can improve the understanding of the influence mechanisms of ion migration in 2D layered perovskite field‐effect transistors.