Large cable vibrations of a cable-stayed bridge have been observed frequently around the world in the past decade. They are very harmful to the bridge. For this reason, a new and smart magneto-rheological (MR) damper first used to mitigate vibrations of cables for the Dongting Lake Cable-stayed Bridge (DLB) is investigated in this paper. MR damper has some obvious properties such as insensitivity to temperature variation and the low power required for operation. Its damping coefficient can be easily adjusted by changing the voltage level applied on the MR damper. In order to demonstrate the effectiveness of cable vibration mitigation, experimental devices including supporting system and excitation system are designed, and tests for cables with MR dampers under different voltage levels and without MR dampers on the DLB are completed. For determining a modal damping ratio which is one of the important parameters of stay cables, a new approach to identification of equivalent modal damping ratios of cables incorporated with the MR dampers is also presented in this paper. Results of field tests show that the MR damper is very effective to vibration mitigation of cables. The approach proposed in this paper is practical and feasible.