Laser powder bed fusion (LPBF) additive manufacturing has successfully facilitated advanced mold fabrication and the incorporation of conformal cooling channels (CCCs), promoting upgrades in the mold industry. H13 steel, known for its wear resistance, high-temperature performance, and cost-effectiveness, is widely used in mold manufacturing. However, H13 steel suffers from poor LPBF formability due to the accumulated residual stresses during the manufacturing process. Substrate preheating is an effective strategy for reducing thermal stress, but it also alters the microstructure, texture, and mechanical properties. In this study, the mechanisms underlying the evolution of microstructure, phase selection and texture at preheating temperatures of 200°C and 500°C (referred to as H13-200 and H13-500, respectively) and their effects on the mechanical response of H13 steel are investigated. The results showed that H13-500 exhibits superior tensile yield strength (1451 ± 5 MPa), comparable to that of conventionally manufactured H13 steel (after forging and heat treatment), and higher than that of H13-200 (1065 ± 78 MPa). H13-200 features a typical martensitic matrix with retained austenite and minor carbides, while H13-500 contains less retained austenite and more carbides. Importantly, the primary BCC phases in H13-500 are martensite and bainite due to their different thermal histories. Additionally, both samples display a special cellular substructure with segregation of C, Cr, and Mo at the cellular boundaries, indicating that the retained austenite first formed during solidification and that bainite/martensite formed in the subsequent cooling process by solid-state transformation. According to Holloman's analysis and strain hardening rate curve, the tensile deformation sequence in H13-200 is identified as "overall elastic deformation → austenite plastic deformation → austenite/martensite plastic deformation." The hard martensite constrained the free plastic deformation of the retained austenite, which improved the tensile yield of H13-200. In the co-deformation stage, the soft retained austenite sustained a much higher strain than did the hard martensite. In contrast, H13-500 follows a sequence of "overall elastic deformation → austenite/bainite plastic deformation → austenite/bainite/martensite plastic deformation." The plastic deformation for H13-500 was mainly concentrated in the austenite/bainite plastic deformation stage. This shift in the deformation sequence and plastic amount highlights that the key phase affecting the tensile performance transitions from retained austenite in H13-200 to bainite in H13-500. These findings suggest that controlling the thermal history during the forming process can effectively improve the mechanical properties of LPBF tool steel.