Mucin glycoproteins contribute to a wide range of cell-surface phenomena. Their dense glycosylation is believed to confer structural rigidity as well as molecular extension beyond the glycocalyx, crucial to interaction with the cellular environment. However, controlled investigations of the relationships between glycosylation, rigidity, and extension of membrane-bound mucins or similar macromolecules are lacking, largely because of the absence of tractable experimental models. We have therefore made use of recently developed synthetic mucin mimetics, in which the core a-GaINAc monosaccharides of natural mucins are conjugated to a lipidated polymer backbone and anchored to fluid, solid-supported lipid membranes, and fluorescence interference contrast microscopy, an optical technique that provides nanometer-scale topographic information about objects near a reflective interface, to measure the orientation of the mucin mimics relative to the membrane plane. Data from two independent probes, fluorophores conjugated directly to the polymer backbone and fluorescent proteins bound to the sugar groups, unexpectedly show that the mucin mimic molecules lie flat along the membrane. Rigidity and core glycosylation are therefore insufficient to ensure molecular projection outward from a membrane surface.