Driven by the increasing demand for passenger safety and weight reduction in the automotive industry, advanced Fe-Mn-Al-C alloys with low density and specific high strength have gained great attention in recent decades. Unfortunately, these materials are prone to hydrogen embrittlement (HE) which impedes their further application. Here we focus on a high-Mn and high-Al lightweight steel (Fe-20Mn-9Al-0.6C wt.%) with an austenite and ferrite two-phase microstructure and elucidate the effects of H-associated decohesion and localized plasticity on its H-induced failure. The HE in this alloy is driven by both, H-induced intergranular cracking along austenite-ferrite phase boundaries and H-induced transgranular cracking inside the ferrite. The former phenomenon is attributed to the H-enhanced decohesion mechanism. For the latter damage behavior, systematic scanning electron microscopy-based characterization reveals that only parts of the transgranular cracks inside ferrite are straight (~52%) and along the {100} cleavage plane. Other such type of cracks shows a distinct deviation from the {100} plane at certain stages of crack propagation, which is associated with a mechanism transition from the H-enhanced decohesion of the ferrite cleavage planes to the H-associated localized plasticity occurring near the propagating crack tip. These mechanisms are further discussed based on a detailed comparison to the nanoindentation results conducted with in-situ H-charging. Those findings can provide some new insights into the boundary conditions of different HE mechanisms in high-strength alloys, their interplay and synergistic effects on damage evolution.