<p>Heat-treated steel alloys with enhanced performance are widely used in engineering applications. However, the long-term effects of prolonged re-tempering on the mechanical and tribological properties of AISI 4140 steel remain unexplored, despite their critical importance for components exposed to severe service conditions. This study systematically investigates the impact of re-tempering at 20&#xa0;h and temperatures from 400&#xa0;°C to 800&#xa0;°C on the microstructure, mechanical properties, and wear behaviour of quenched and tempered AISI 4140 steel. Microstructural investigations revealed that as the re-tempering temperature (<i>T</i>_r) increased, the surface morphology of Fe₃C precipitate changed from rod-shaped (400&#xa0;°C) to globular-shaped (700&#xa0;°C). Besides, pearlitic phase formation occurred when the recrystallization temperature is above the austenitizing point, i.e. <i>T</i>_r at 800&#xa0;°C. Notably, increasing the re-tempering temperature resulted in a significant decrease in yield strength (up to 53%) and ultimate tensile strength (up to 31%), while elongation improved by up to 48%. A decrease in hardness (up to 25%) was observed till 700&#xa0;°C (except 500&#xa0;°C), and it was attributed to the carbide particles dissolving at high temperature. Temper embrittlement, on the other hand, causes an increase in hardness at <i>T</i>_r = 500&#xa0;°C due to the formation of secondary phases (Cr₂₃C₆). Fracture morphology revealed that at 500&#xa0;°C, the failure mode was brittle, whereas, at other <i>T</i>_r, the failure mode was ductile, with the creation of voids in the microstructures. Furthermore, with increasing <i>T</i>_r, a gradual increase in retained austenite concentration was found. Tribological behaviour indicated better wear resistance at higher <i>T</i>_r. This study implies that re-tempering steel for an extended length of time had a significant effect on its mechanical properties, resulting in changed microstructural features.</p>