Background Few data exist on the association between strength training and mortality rates. We sought to examine the association between strength training and all‐cause, cardiovascular disease, and cancer mortality. Methods and Results Beginning in 2001 to 2005, 28 879 women throughout the United States (average baseline age, 62.2 years) from the Women's Health Study who were free of cardiovascular disease, diabetes mellitus, and cancer reported their physical activities, including strength training. During follow‐up (average, 12.0 years) through 2015, investigators documented 3055 deaths (411 from cardiovascular disease and 748 from cancer). After adjusting for covariables, including aerobic activity, time in strength training showed a quadratic association with all‐cause mortality (P=0.36 for linear trend; P<0.001 for quadratic trend); hazard ratios across 5 categories of strength training (0, 1–19, 20–59, 60–149, and ≥150 min/wk) were 1.0 (referent), 0.73 (95% confidence interval, 0.65–0.82), 0.71 (0.62–0.82), 0.81 (0.67–0.97), and 1.10 (0.77–1.56), respectively. A significant quadratic association was also observed for cardiovascular disease death (P=0.007) but not cancer death (P=0.41). Spline models also indicated a J‐shaped nonlinear association for all‐cause mortality (P=0.020); the point estimates of hazard ratios were <1.00 for 1 to 145 min/wk of strength training, compared with 0 min/wk, whereas hazard ratios were >1.00 for ≥146 min/wk of strength training. However, confidence intervals were wide at higher levels of strength training. Conclusions Time in strength training showed a J‐shaped association with all‐cause mortality in older women. A moderate amount of time in strength training seemed beneficial for longevity, independent of aerobic activity; however, any potential risk with more time (≈≥150 min/wk) should be further investigated.