The effects of silicon and nickel addition and hot extrusion process on the microstructure and mechanical properties of in-situ magnesium-matrix composites were studied. By increasing the amount of Si, the volume fraction of primary Mg2Si particles increased toward more pronounced long dendritic forms, which resulted in the deterioration of as-cast tensile properties. A new primary intermetallic phase containing Ni, Si, and Mg appeared in the microstructure upon the addition of Ni to Mg-Si system, where its effect on the as-cast tensile properties was insignificant. However, after extrusion process, the tensile strength of composites enhanced with increasing the amount of primary phases. Moreover, major enhancements in mechanical properties, tensile toughness, and fracture surface appearance were observed for extruded composites compared with the as-cast counterparts. These results can shed light on the capability of these alloys for potential applications.