This study examines the microstructural evolution and mechanical properties of P21 tool steel fabricated via laser powder bed fusion (LPBF). A fine martensitic matrix with minimal retained austenite forms near the top due to rapid solidification, transitioning to columnar prior austenite grains with aligned martensitic laths mid-build. Toward the bottom, microstructures coarsen, showing lath thickening and signs of tempering from thermal accumulation and cyclic reheating. These variations significantly impact mechanical behavior. P21 specimens exhibited average ultimate tensile strengths of 902 ± 20 MPa and 843 ± 5 MPa, with elongations of 23.50 ± 1.00 % and 24.70 ± 1.30 %, in the horizontal and vertical orientations, respectively. The results highlight critical structure-property relationships in LPBF-processed P21 steel and offer insights for optimizing performance in tooling applications.