Variations in lower bainite and martensite phase configurations through thickness play an important role in achieving an optimum combination of strength and toughness for thick high-strength low-alloy (HSLA) steel plates. A comprehensive understanding of the hardness and microstructural evolution of tempered lower bainite and martensite (LB/M) mixtures contributes to the adjustment of the quenching and tempering parameters to further control HSLA plate deformation during manufacturing. Therefore, this study focused on the tempering behavior of the mixed LB/M microstructure and compared this behavior with those of the singular martensite and lower bainite phases. Results have shown that the hardness of LB/M microstructures follows the rule of mixtures. Hardness declines from singular martensite to the LB/M microstructure and further decreases to singular lower bainite during short-term tempering, whereas an opposite hardness decreasing trend is observed during long-term tempering. Carbide coarsening leads to a decrease in hardness during short-term tempering, where the coarsening of martensitic and lower bainitic carbides in the LB/M microstructure is consistent with that of singular martensitic and bainitic carbides. Furthermore, the coarsening of laths and carbides in the LB/M mixture is similar to that in the singular lower bainitic microstructure for long-term tempering, where lower bainitic carbides are more stable than martensitic carbides.