Isonitrile (also termed isocyanide) natural products exhibit potent antimicrobial activities, but little is known about their molecular targets. Here, we focus on the so far neglected group of monoisonitriles to gain further insights into their antimicrobial mode of action (MoA). Screening a focused monoisonitrile library revealed a potent S. aureus growth inhibitor with a different MoA compared to previously described diisonitrile antibiotics. Chemical proteomics via competitive cysteine reactivity profiling, unraveled covalent modifications of two essential metabolic enzymes involved in the fatty acid biosynthetic process (FabF) and the hexosamine pathway (GlmS) at their active site cysteines. In-depth studies with the recombinant enzymes demonstrated concentration-dependent labeling, covalent binding to the catalytic site and corresponding functional inhibition by the isocyanide. Thermal proteome profiling and full proteome studies of compound-treated S. aureus further highlighted the destabilization and dysregulation of proteins related to the targeted pathways. The here described novel, covalent isocyanide MoA highlights the versatility of the functional group, making it a great tool and out-of-the-box starting point for the development of innovative antibiotics.