An active multi-core fiber (MCF) is used as the basis for a monolithic fiber laser. After characterization of the Yb-doped MCF, the effect of multi-mode interference (MMI) between competing supermodes within the laser is explored. It is shown that the laser characteristics are influenced by the MMI spectrum, as the maximum laser power and efficiency occur for MMI maxima. The temporal characteristics of the laser are also examined, and the presence of a weak saturable absorber is observed. By increasing the cavity length via additional single-mode fiber, stable mode-locking is observed for cavities longer than 22 meters.