Spherical micro-Ti particle (TiP)-reinforced AZ91 magnesium alloy composites were fabricated by semi-solid stirring assisted ultrasonic vibration, which were then subjected to hot extrusion. The microstructure results showed that the addition of Ti particles refined the grain size and decreased the texture intensity of the as-extruded AZ91 alloy. An Al3Ti phase with a thickness of 100 nm formed at the Ti/Mg interface, which had a non-coherent relationship with the magnesium matrix. The as-extruded 1 vol.% TiP/AZ91 composite exhibited the best comprehensive mechanical properties, with yield strength, ultimate tensile strength, and elongation at break of 366 MPa, 456 MPa, and 14.6%, respectively, which were significantly higher than those of the AZ91 alloy. Therefore, the addition of Ti particles can improve the strength and ductility of the AZ91 alloy, demonstrating the value of magnesium matrix composites for commercial applications.