This paper reports a novel magnetorheological fluid (MRF)-based damper, which synergizes the attributes of variable stiffness and damping through the compact assembly of two MRF damping units and a spring. The magnetic field densities of the two damping units were analyzed. After the prototype of the new MRF damper, a hydraulically actuated MTS machine was used to test the damper's performance, including stiffness variability and damping variability, amplitudedependent responses and frequency-dependent responses. A new mathematical model was developed to describe the variable stiffness and damping MRF damper. The successful development, experimental testing and modeling of this innovative variable stiffness and damping MRF damper make the true design and implementation of the concept of variable stiffness and damping feasible.