The increased use of magnesium alloys with improved mechanical properties is a prominent strategy towards increasing fuel efficiency of vehicles and decreasing emissions. This study investigates the grain refining efficiency and fading of MgB2 micro- and nano-particle added Pure Mg and AZ91E. Addition of micro and nano-sized MgB2 provided a reduction in grain size for Pure Mg and AZ91E. Enhanced heterogeneous nucleation and grain growth restriction was believed to be the source of refinement. Fading was observed for both Pure Mg and AZ91E, with the nano-particle added castings showing an increased resistance. The elongation of the Pure Mg samples showed improvements, whereas no improvements for UTS and YS was seen. The improved ductility was believed to be due to the grain refinement and coefficient of thermal expansion mismatch. The AZ91E samples did not show improvements in mechanical properties. This was believed to be due to stress concentration from Al-Mn intermetallics.