Conventional slow and fast light (SFL) systems are based on active or nonlinear media or photonic band-gap systems. Radiofrequency (RF) interferometric systems were recently explored for sustaining fast and slow light, owing to their simple structures and passive operation. This Letter introduces a simple, effective, and efficient SFL system based on a linear and passive microwave-photonic (MWP) fiber optic Mach–Zehnder interferometer (MZI). By optically tuning the light transmission ratio of the two fiber beams of the MZI (i.e., the optical carrier), we demonstrate that both delay and advancement of a RF signal (i.e., the envelope signal) can be realized. The magnitude of the signal group delay can also be adjusted using the optical tuning method. Therefore, high-degree manipulation of signal group delay can be achieved using the proposed MWP fiber optic MZI system.