Immune cells need to sustain a state of constant alertness over a lifetime. Yet, little is known about the regulatory processes that control the fluent and fragile balance that is called homeostasis. Here we demonstrate that JAK-STAT signaling, beyond its role in immune responses, is a major regulator of immune cell homeostasis. We investigated JAK-STAT-mediated transcription and chromatin accessibility across 12 mouse models, including knockouts of all STAT transcription factors and of the TYK2 kinase. Baseline JAK-STAT signaling was detected in CD8+ T cells and macrophages of unperturbed mice – but abrogated in the knockouts and in unstimulated immune cells deprived of their normal tissue context. We observed diverse transcription-regulatory programs, including gene regulation by STAT2 and IRF9 independent of STAT1. In summary, our large-scale dataset and integrative analysis of JAK-STAT mutant and wildtype mice uncovered a crucial role of JAK-STAT signaling in unstimulated immune cells, where it contributes to a poised epigenetic and transcription-regulatory state and helps prepare these cells for rapid response to immune stimuli.