<p>This paper presents a semi-active control strategy that integrates time-delayed velocity feedback with magnetorheological (MR) dampers to suppress large-amplitude sling vibrations in long-span cable-supported bridges. To address the limitations of conventional active control methods, such as excessive energy consumption and limited stability, a coupled vibration model of the sling–MR damper system is developed by incorporating a time-delayed velocity feedback mechanism and exploiting the adjustable damping characteristics of MR dampers. In addition, a mapping algorithm is proposed to address the issue of negative current values in the damper. Numerical simulations reveal that the time-delay produces periodic effects on vibration suppression performance, with nonlinear influences becoming particularly significant as the frequency ratio approaches resonance peaks. With optimized time-delay selection in combination with MR dampers, the system achieves up to about a 30% reduction in the root-mean-square vibration amplitude while avoiding instability. The proposed approach significantly improves the efficiency of sling vibration control, providing an energy-efficient and reliable semi-active mitigation solution.</p>