Thin-walled tubes are widely used as energy absorbers due to their high crashworthiness performance. This study aims to evaluate the effect of plastic forming history carried out for thickness optimization on crashworthiness performance of thin-walled square tube (TWST). Within the scope of the study, a series of numerical analyzes were conducted for the TWST using commercial finite element (FE) software. In order to determine the individual effect of optimization, the FG thickness of TWST was first obtained using LS-OPT software. Later, thickness gradient was achieved by performing a rolling process to consider individual plastic forming effects. Afterwards, a deep drawing process was carried out to shape TWST by considering springback and trimming effects. Finally, crash responses were obtained under axial high-velocity impact loading to determine the coupled effect of optimization and plastic forming on the crashworthiness performance of the TWST. It was determined that the coupled effect, which takes plastic forming and optimization effects into account, reduced the peak crush force of the TWST by 24% and increased the absorbed energy value by 39%. The results obtained from this study showed that coupled effect of optimization and plastic forming processes has a significant effect on the crashworthiness performance of the TWST, and otherwise, either overestimated or underestimated results are obtained.