Abstract: Custom 465 (C465) is a martensitic stainless steel known for its high strength, toughness, and corrosion resistance, widely used in aerospace, automotive, and medical industries. However, limited work has been conducted on its additive manufacturing (AM) and no dedicated heat treatments have been developed for additively manufactured C465 to optimize its strength–ductility trade-off. In this work, the C465 was fabricated via laser powder bed fusion. The effect of hot isostatic pressing, solid solution, cryogenic treatment (−78.5°C), and aging on the composition homogenization, austenite-to-martensite transition, and Ni₃Ti precipitation were systemically investigated. The atom probe tomography analysis reveals that Mo atoms accumulate on Ni₃Ti precipitate surfaces and inhibits the Ni₃Ti growth, contributing to the enhanced strength of C465. The modified heat treatment for additively manufactured C465 reaches comparable tensile strength with the wrought counterpart, yielding an ultimate tensile strength of 1773 MPa, yield strength of 1686 MPa, and elongation of 6.5%. A yield strength calculation model was proposed and validated with measured strength under various heat treatments, providing valuable insight for heat treatment design towards diverse industrial applications.