In this study, the effect of microstructure evolution on the impact properties of high-strength low-alloy (HSLA) steel plates was investigated. Two positions (surface and inner) throughout the thickness direction of a block of the selected steel were comprehensively investigated using mechanical tests and microstructural characterization techniques. Considering the combination of structural units, carbides, and dislocation density, the best performance of strength and toughness were found in the surface and inner samples, respectively. The exceptional ductile–brittle transition temperature (DBTT) behavior was attributed to the carbide spacing distribution caused by the adopted thermo-mechanically controlled processing (TMCP).