Lysine-specific demethylase 1 (LSD1) is a promising therapeutic target, especially in cancer treatment. Despite several LSD1 inhibitors being discovered for the cofactor pocket, none are FDA-approved. We aimed to develop stabilized peptides for irreversible LSD1 binding, focusing on unique cysteine residue Cys360 in LSD1 and SNAIL1. We created LSD1 C360-targeting peptides, like cyclic peptide S9-CMC1, using our Cysteine-Methionine cyclization strategy. S9-CMC1 effectively inhibited LSD1 at the protein level, as confirmed by MS analysis showing covalent bonding to Cys360. In cells, S9-CMC1 inhibited LSD1 activity, increasing H3K4me1 and H3K4me2 levels, leading to G1 cell cycle arrest and apoptosis and inhibiting cell proliferation. Remarkably, S9-CMC1 showed therapeutic potential in A549 xenograft animal models, regulating LSD1 activity and significantly inhibiting tumor growth with minimal organ damage. These findings suggest LSD1 C360 as a promising site for covalent LSD1 inhibitors' development.