OBJECTIVES:To prospectively examine associations between accelerometer-measured physical activity (PA) and mortality in older women, with an emphasis on light-intensity PA. DESIGN:Prospective cohort study with baseline data collection between March 2012 and April 2014. SETTING:Women's Health Initiative cohort in the United States. PARTICIPANTS:Community-dwelling women aged 63 to 99 (N = 6,382). MEASUREMENTS:Minutes per day of usual PA measured using hip-worn triaxial accelerometers, physical functioning measured using the Short Physical Performance Battery, mortality follow-up for a mean 3.1 years through September 2016 (450 deaths). RESULTS:When adjusted for accelerometer wear time, age, race-ethnicity, education, smoking, alcohol, self-rated health, and comorbidities, relative risks (95% confidence intervals) for all-cause mortality across PA tertiles were 1.00 (referent), 0.86 (0.69, 1.08), 0.80 (0.62, 1.03) trend P = .07, for low light; 1.00, 0.57 (0.45, 0.71), 0.47 (0.35, 0.61) trend P < .001, for high light; and, 1.00, 0.63 (0.50, 0.79), 0.42 (0.30, 0.57) trend P < .001, for moderate-to-vigorous PA (MVPA). Associations remained significant for high light-intensity PA and MVPA (P < .001) after further adjustment for physical function. Each 30-min/d increment in light-intensity (low and high combined) PA and MVPA was associated, on average, with multivariable relative risk reductions of 12% and 39%, respectively (P < .01). After further simultaneous adjusting for light intensity and MVPA, the inverse associations remained significant (light-intensity PA: RR = 0.93, 95% CI = 0.89-0.97; MVPA: RR = 0.67, 95% CI = 0.58-0.78). These relative risks did not differ between subgroups for age or race and ethnicity (interaction, P ≥ .14, all). CONCLUSION:When measured using accelerometers, light-intensity and MVPA are associated with lower mortality in older women. These findings suggest that replacing sedentary time with light-intensity PA is a public health strategy that could benefit an aging society and warrants further investigation.