Polymer-modified cementitious repair mortars have been used extensively in recent years for concrete repairs in conjunction with electrochemical protection systems. The performance improvements of polymer additions to cementitious mortars generally come at the expense of increased mortar resistivity, an important consideration in the selection of repair mortars particularly when used in conjunction with electrochemical protection systems, such as impressed current cathodic protection and galvanic anode systems. In this paper, four-point Wenner probe resistivity tests of four commercially available mortars marketed as ‘low resistivity’ polymer-modified repair mortars were carried out over a period of 18 months. The experiment results indicated substantial increases in resistivity over time for all mortars in saturated and outdoor exposure conditions, which were beyond the short-term resistivity data of 28 days presented in manufacturers’ technical data sheets and perceived to be the long-term maximum mortar resistivity. The outcome of this paper suggests that polymer-modified mortar resistivity increases substantially over time. The increase of the repair mortar resistivity when used in conjunction with electrochemical protection systems may have a considerable impact on the performance of these systems.