The Co-based superalloy Mar M 509, known for its high-temperature oxidation and hot corrosion resistance, is processed via laser powder bed fusion (LPBF). Microstructure and mechanical properties of Mar M 509 in as-printed (As-P) and heat-treated (HT) states are compared based on two build orientations (longitudinal (L) and transverse (T)) to establish structure-property links with heat treatment. The As-P condition displays a distinct cellular microstructure (500–600 nm) with 50–60 nm carbide particles adorning cell boundaries. Longitudinal (L) build has columnar grains (8–35 μm along the major axis) with a grain aspect ratio of 4, while transverse (T) orientation exhibits equiaxed, bimodal microstructure (5–10 μm and 15–25 μm grain sizes). Strong <001> texture is noted in L. Mechanical properties at room temperature differ between L and T; T (569 ± 12 HV) has 15% higher hardness compared to L (489 ± 18 HV) and 34% higher 0.2% yield strength (YS), but 30% lower elongation than L. Post a short heat treatment cycle at 1250 °C, weld bead structure and cell boundaries break down. Both L (25–33 μm along the major axis) and T orientations (5–42 μm) experience grain growth, and carbides coarsen (250–350 nm). Post-heat treatment, dislocation density decreases, indicating recrystallization; lattice parameter of matrix reduces, implying solute depletion contributing to carbide enrichment. Yield strength drops from 860 MPa to 740 MPa in L and from 1150 MPa to 840 MPa in T, with ductility rising from 14% to 23% in L.