AbstractThe migration of ionic defects and electrochemical reactions with metal electrodes remains one of the most important research challenges for organometal halide perovskite optoelectronic devices. There is still a lack of understanding of how the formation of mobile ionic defects impact charge carrier transport and operational device stability, particularly in perovskite field‐effect transistors (FETs), which tend to exhibit anomalous device characteristics. Here, the evolution of the n‐type FET characteristics of one of the most widely studied materials, Cs_0.05FA_0.17MA_0.78PbI_3, is investigated during repeated measurement cycles as a function of different metal source–drain contacts and precursor stoichiometry. The channel current increases for high work function metals and decreases for low work function metals when multiple cycles of transfer characteristics are measured. The cycling behavior is also sensitive to the precursor stoichiometry. These metal/stoichiometry‐dependent device non‐idealities are correlated with the quenching of photoluminescence near the positively biased electrode. Based on elemental analysis using electron microscopy the observations can be understood by an n‐type doping effect of metallic ions that are created by an electrochemical interaction at the metal–semiconductor interface and migrate into the channel. The findings improve the understanding of ion migration, contact reactions, and the origin of non‐idealities in lead triiodide perovskite FETs.