Of the recent developments in legged robotics, compliance control of legs has potential to make a huge impact in the transition from simple mechanisms to life-like machines. Given the nature of legged locomotion in humans and animals involves such compliance control, this is a step forward in terms of the artificial embodiment of the biological traits that make these living beings superior to the robots we have today. Taking a literal Rolling Spring Loaded Inverted Pendulum (R-SLIP) morphology, this work proposes a magnetorheological-fluid-centric (MRF) variable stiffness leg, aiming to further investigate this area and take advantage of the improved energy efficiency and gait stability made possible through leg stiffness control. With the potential for adaptive tunability of leg stiffness through semi-active control, the design is shown to be capable of a maximum stiffness increase of 257%, with behaviour predicted by a force model describing the mechanism. In subsequent dynamic locomotion testing was conducted and the testing results demonstrate that the new leg is able to reduce cost-of-transport (CoT) and verified the potential of the MRF leg on improving the locomotive performance.