With advances in cryogenic engineering, increasingly stringent performance requirements are imposed on cryogenic structural alloys. Conventional heat treatments are often inadequate to satisfy the demanding performance requirements of 18Ni maraging steel used in aerospace and other high-end applications, creating a need for alternative heat-treatment strategies. Although deep cryogenic treatment has been reported to enhance the properties of various steels, its effects on maraging steels remain insufficiently understood. Accordingly, this study investigates the influence of cryogenic treatment temperature and duration on the microstructure and mechanical properties of 18Ni(200) maraging steel. The material was subjected to a combined process comprising solution treatment, cryogenic treatment, and aging. After solution treatment at 800 °C for 1 h, cryogenic treatments were performed at −78 °C for 4, 8, and 12 h, and at −196°C for 12 h, followed by aging at 550 °C for 4 h. Mechanical properties were evaluated using tensile testing and hardness measurements, while microstructural evolution was characterized by XRD, EBSD, and TEM. The results indicate that deep cryogenic treatment not only promotes the transformation of reversed austenite into martensite and refines martensitic laths, but also induces lattice distortion in martensite. This distortion is manifested by high stress concentrations and a high dislocation density, which increase the internal energy of the material. During subsequent aging, the release of this stored energy enhances the diffusion driving force of alloying elements, thereby promoting the formation of fine precipitates with a high number density. Among the investigated conditions, the specimen cryogenically treated at −78 °C for 12 h exhibited the best overall performance, achieving a yield strength of 1778.9 MPa, a hardness of 484.9 HV, and an elongation of 7.2%. In comparison, specimens treated at −196 °C showed a lower content of reversed austenite after aging. The lower cryogenic temperature induced greater lattice strain in the martensite, leading to the accumulation of a large amount of internal (potential) energy within the material. During subsequent aging, this stored energy was rapidly released, leading to strain recovery, a reduction in dislocation density, and a decrease in both the size and number density of precipitates. These effects ultimately resulted in lower strength and ductility compared with specimens treated at −78 °C. At −78 °C, increasing the cryogenic treatment duration progressively reduced the content of reversed austenite and refined its morphology. Concurrently, the fraction of low-angle grain boundaries and the precipitate size decreased. These microstructural evolutions enhanced grain-refinement and dispersion strengthening, resulting in significant increases in the strength and hardness of the steel. Overall, deep cryogenic treatment effectively optimizes the microstructure and mechanical properties of 18Ni(200) maraging steel by promoting martensite refinement and a favorable precipitate distribution. However, excessively low cryogenic temperatures may accelerate strain recovery during aging and diminish strengthening effects. This study provides practical guidance for designing cryogenic treatment protocols for high-performance maraging steels intended for cryogenic applications.