Abstract: Multistage heat treatment involving quenching (Q), lamellarizing (L), and tempering (T) is applied to marine 10Ni5CrMoV steel. The microstructure and mechanical properties were studied by multiscale characterizations, and the kinetics of reverse austenite transformation, strain hardening behavior, and toughening mechanism were further investigated. The lamellarized specimens possess low yield strength but high toughness, especially cryogenic toughness. Lamellarization leads to the development of film-like reversed austenite at the martensite block and lath boundaries, refining the martensite structure and lowering the equivalent grain size. Kinetic analysis of austenite reversion based on the JMAK model shows that the isothermal transformation is dominated by the growth of reversed austenite, and the maximum transformation of reversed austenite is reached at the peak temperature (750°C). The strain hardening behavior based on the modified Crussard–Jaoul analysis indicates that the reversed austenite obtained from lamellarization reduces the proportion of martensite, significantly hindering crack propagation via martensitic transformation during the deformation. As a consequence, the QLT specimens exhibit high machinability and low yield strength. Compared with the QT specimen, the ductile–brittle transition temperature of the QLT specimens decreases from −116 to −130°C due to the low equivalent grain size and reversed austenite, which increases the cleavage force required for crack propagation and absorbs the energy of external load, respectively. This work provides an idea to improve the cryogenic toughness of marine 10Ni5CrMoV steel and lays a theoretical foundation for its industrial application and comprehensive performance improvement.