Dissecting the mechanism of action of surface-tethered antimicrobial and immunomodulatory peptides is critical to the design of optimized anti-infection coatings on biomedical devices. To address this, we compared the biomembrane interactions of host defense peptide IDR-1010cys (1) in free form, (2) as a soluble polymer conjugate, and (3) with one end tethered to a solid support with model bacterial and mammalian lipid membranes. Our results show that IDR-1010cys in all three distinct forms interacted with bacterial and mammalian lipid vesicles, but the extent of the interactions as monitored by the induction of secondary structure varied. The enhanced interaction of surface-tethered peptides is well correlated with their very good antimicrobial activities. Our results demonstrate that there may be a difference in the mechanism of action of surface-tethered versus free IDR-1010cys.