Short helical antimicrobial peptides forming inter-molecular disulfide bonds are selected against in nature, and were utilized here to design switchable antimicrobials via the formation of functional supramolecular fibrils. Specifically, using the available structural information on the stable fibril-forming human LL-37(17-29), we designed cysteine mutations and demonstrated position-dependent controllable antibacterial activity, mediated by their disulfide-dependent self-assembly into ordered fibrils, which proved sensitive to reducing conditions. The crystal structure of the LL-37(17-29) bearing a I24C substitution, located in a critical structural position, revealed disulfide-bonded dimers that further assembled into a fibrillar structure of densely packed helices. The native and mutant peptides both featured a fibril surface with zigzagged hydrophobic and positively charged belts, which likely underlie interactions with bacterial membranes. Yet, they differed in their helical packing arrangement, which corresponded with different levels of activity, with only the mutant being susceptible to reducing conditions. The presented findings promise to advance the design of novel antimicrobials resistant to harsh conditions for coating of surfaces susceptible to pathogens.