We report on a new electrochemical sensor for Concanavalin A. It is based on blocking the surface of plasticized PVC membranes that were covalently modified with D-mannose using click chemistry. The interaction of D-mannose with Concanavalin A on the surface perturbs the flux of a marker ion for which the ion-selective membrane is responsive, and this results in a change in the electrochemical signal. The sensor was characterized using a variety of electrochemical protocols, and results were confirmed by quartz crystal microbalance experiments. The lowest limit of detection (10 μg mL−1) was obtained using a membrane containing a cation exchanger and tetrabutylammonium ion as the marker ion.