The design of metal matrix composites predominantly focusses on synergistically integrating the intrinsic properties of matrix and reinforcing phase aiming to achieve outstanding strength and ductility simultaneously. In metal matrix composites, the reinforcing phase invariably faces challenges such as agglomeration and non-uniform distribution, resulting in a significant reduction in mechanical properties. Here, we propose a method to overcome this barrier through an in-situ alloying of laser powder bed fusion i.e., redistributing the reinforcing phase (Cr2B boride) in medium entropy alloy matrix (CoCrNi). The in-situ alloying can be achieved by a mixture of CoCrNi powders and B4C particles. After remelting, a nano-sized core-shell structure is developed by establishing “soft core” of CoCrNi matrix embedded with “hard shell” of Cr2B boride. Such nano-sized core-shell structure mainly contributes to a yield strength (867 ± 1.6 MPa), a high tensile strength (1200 ± 13 MPa) and a decent ductility (17 ± 0.5%) of the CoCrNi/Cr2B composite due to the load bearing strengthening and hetero-deformation induced hardening. Additionally, the crack propagation can be suppressed by the CoCrNi matrix/Cr2B interface. The insights provide a new design strategy of in-situ formation of the nano-sized core-shell structure to develop the metal matrix composites with high strength and ductility. Display omitted