Human guanylate-binding protein 1 (hGBP1) is a key player in innate immunity and fights diverse intracellular microbial pathogens. Its antimicrobial functions depend on hGBP1's GTP binding- and hydrolysis-induced abilities to form large, structured polymers and to attach to lipid membranes. Crucial for both of these biochemical features is the nucleotide-controlled release of the C terminally located farnesyl moiety. Here, we address molecular details of the hGBP1 membrane binding mechanism by employing recombinant, fluorescently labeled hGBP1, and artificial membranes. We demonstrate the importance of the GTPase activity and the resulting structural rearrangement of the hGBP1 molecule, which we term the open state. This open state is supported and stabilized by homodimer contacts involving the middle domain of the protein and is further stabilized by binding to the lipid bilayer surface. We show that on the surface of the lipid bilayer a hGBP1 monolayer is built in a pins in a pincushion-like arrangement with the farnesyl tail integrated in the membrane and the N-terminal GTPase domain facing outwards. We suggest that similar intramolecular contacts between neighboring hGBP1 molecules are responsible for both polymer formation and monolayer formation on lipid membranes. Finally, we show that tethering of large unilamellar vesicles occurs after the vesicle surface is fully covered by the monolayer. Both hGBP1 polymer formation and hGBP1-induced vesicle tethering have implications for understanding the molecular mechanism of combating bacterial pathogens. DATABASES: Structural data are available in RCSB Protein Data Bank under the accession numbers: 6K1Z, 2D4H.