In this work, a novel TiC nanoparticle (0.5 wt %) reinforced Mg-2.9Zn-1.1Ca-0.5Mn nanocomposite, with a mixed grain microstructure exhibiting high strength and targeted for biocompatible/structural applications was successfully prepared by ultrasonic assisted semi-solid stirring and extrusion at ultra-slow speeds of 1, 0.1 and 0.01 mm/s. The experimental results revealed that the morphology of the eutectic Ca2Mg6Zn3 phase in the as-cast nanocomposite changed from plate-like to lamellar as a low mass fraction of TiCp was added. Both dynamically recrystallized grains and precipitates were gradually refined with decreasing extrusion speed. The finest recrystallized grains (∼0.46 μm), with a high volume fraction (∼4.3%) of fine precipitates, appeared after extrusion at 0.01 mm/s. The refined grain structure was not only due to dynamic recrystallization, but also the synergistic pinning effects from nano-TiCp as well as precipitated MgZn2 and α-Mn particles. The superior ultimate tensile strength (∼410.3 MPa), yield strength (∼384.5 MPa) and elongation to failure (∼4%) were obtained in the nanocomposite extruded at the slowest speed (0.01 mm/s) and had the potential to serve as a candidate material in orthopaedic applications. The improved strength was mainly related to grain refinement, thermal expansion effects and Orowan strengthening. Grain refinement, in particular, contributed to the largest strengthening increment. High tensile toughness of ∼66.6 KJ mm−3 was achieved for the nanocomposite extruded at a speed of 1 mm/s. Further, it exhibited a high strain hardening rate, θ, at stage IV. The fracture surface exhibited abundant dimples and consistent with high ductility. Remnant coarse Ca2Mg6Zn3 particles acted as crack initiators under high applied stress in tension, leading to structural failure.