Metastability-based high entropy alloy design opens a new strategic path for designing high-strength materials. However, high strength is always coupled with poor damage tolerance under cyclic loading conditions (fatigue). To overcome this drawback, here we present grain-refined Fe42Mn28Cr15Co10Si5 exhibiting significantly high fatigue strength as compared with leading transformation induced plasticity steels upon friction stir processing. The enhanced fatigue behavior is attributed to the metastability-promoted γ→ϵ transformation that caused local variation in work-hardening activity near the crack tip, and subsequent crack branching. Thus, decreased γ phase stability assisted not only in attaining strength but also in making the alloy fatigue-resistant. © 2018, © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.