An improved cumulative damage criterion is proposed for preloaded threaded fasteners. The proposed criterion takes into account the effect of bolt mean stress, damage history, and the loading sequence. Baseline mean stress-adjusted fatigue curves are first established for two sizes of A286 steel bolts with round socket heads. Fatigue curves are subsequently lowered due to calculable partial damage that each step of a loading spectrum. Theoretical results are experimentally validated using a three-stage loading spectrum that is applied in a low-to-high (L–H) or high-to-low (H–L) loading sequence.