Working towards full implementation of semi-active MR dampers to over 200 cables in the cable-stayed Dongting Lake Bridge, a few selected stay cables were installed with both MR dampers and vibration sensors for trial testing before full implementation. This paper reports the field vibration tests of a typical stay cable of 115 m long before and after being installed with MR dampers. The field vibration tests were conducted by using ambient vibration excitation, and sinusoidal excitation followed by free vibration decay, respectively. The resonant frequencies and equivalent modal damping of the first twenty in-plane modes of the cable with and without MR dampers are identified. The relationship between the equivalent modal damping ratio and the applied voltage strength to MR dampers is experimentally determined. The test results show that the modal damping ratios of the free cable without dampers conform to a combined Rayleigh and frequency-independent damping model, and are almost unvaried with vibration amplitude within the tested range. Installation of MR dampers results in a slight change of resonant frequencies in comparison with the free cable, but variation in applied voltage almost does not affect the resonant frequencies. The equivalent modal damping ratios of the cable-damper system are found to be dependent on the installation location of MR dampers, voltage strength applied to the dampers, and the cable vibration level. The optimal voltage input, which achieves maximum system damping, is obtained for different modes under different vibration amplitudes. With optimal voltage applied to the dampers, the resulting system damping should be high enough to suppress both wind-rain-induced vibration and wake galloping of cables.