The microstructures and mechanical properties of a newly developed Mg-3.6Zn-0.6Y-0.2Ca matrix composite containing SiC particle before and after extrusion were investigated. The grain sizes of the as-extruded composites were significantly decreased as compared to the as-homogenized composite before extrusion. With increasing the extrusion speed or extrusion temperature, the size of recrystallized grains increased while the volume fraction slightly increased. The size of dynamic precipitated phases increased and the volume fraction decreased with increasing the extrusion speed. With an increase in extrusion speed from 0.01 mm/s to 0.1 mm/s at 230 °C or extrusion temperature from 190 °C to 230 °C at 0.1 mm/s, the yield strength and ultimate tensile strength decreased while the elongation increased. The yield strength, ultimate tensile strength and elongation of composite extruded at optimized extrusion parameters (at 190 °C at 0.1 mm/s) were 312.0 MPa, 347.3 MPa and 6.6%, respectively. The contribution of grain refinement strengthening on the strength increment was higher relative to the thermal expansion effect and precipitates strengthening.