The ambient relative humidity is known to significantly affect mechanical tribological behavior, especially in the presence of static electric contact. When both tribological contact and electrical conduction occur during rolling, the detailed influences of the humidity on such current carrying tribo-contacts remain unclear. In this mechanistic study, the tribological/conductive properties of Cu/Cu friction couples were investigated using a disc-on-disc rolling contact tribometer at different relative humidity levels. It was found that the friction coefficient increased sharply at higher humidity, and that could be attributed to the meniscus force. With increasing humidity, the wear mechanism transformed from a mild form of adhesive and abrasive wear to more severe, lamellar fatigue wear. Energy-dispersive X-ray spectrometry showed that vapor took part in two types of oxidation on the rolling electric contact surfaces: (i) tribo-oxidation and (ii) electrochemical oxidation. The oxides produced on the surfaces led to high electrical resistance, which deteriorated the conductivity of Cu/Cu rolling pairs. These results may provide a deeper understanding of the failure properties of the electro-conductive rotary joints in open air environments.