The effects of casting conditions and hot deformation processing on the mechanical properties of aluminum and magnesium cast composites were determined. A356 Al-20 vol.% SiCp composite ingot from Dural Composite Corporation was either squeeze cast under 140 MPa pressure or sand cast into a cylindrical shape. Both castings were subsequently hot worked via hot swaging to reductions of 33% to 95%. The AZ91 Mg-20 vol.% SiCp composites were prepared at Case Western Reserve University followed by either permanent mold and sand casting techniques to produce keel bars or squeeze casting at 140 MPa pressure. The squeeze cast Mg-SiCp cylinders were additionally hot extruded at 12:1 reduction ratio. Tensile strengths of A356 composite in the T6 condition improved from 235 to 280 MPa with squeeze casting: the tensile strength further increased to 339 MPa with hot deformation processing. Elongation in these composites improves from values of less than 0.5% in the cast condition to values in excess of 8% with 95% hot reduction. Changes in the microstructure of the processed materials accompanied the strength improvement. Tensile strengths of the cast AZ91 Mg composites in the T6 condition (e.g. 295 MPa) were similar for all casting conditions employed while hot extrusion improved the strength (e.g. 432 MPa). The elongation values for the Mg composites were approximately 1.5% elongation for all casting techniques and improved to 3.5% with hot extrusion. Elongations in excess of 5% have been obtained by a shorter aging heat treatment.