BACKGROUND: Physical activity (PA) levels vary across specific population groups, contributing to health inequalities. Little is known about how local authority leisure centers contribute to population PA and whether this differs by age, sex, or socioeconomic group. METHODS: The authors calculated weekly leisure center-based moderate/vigorous PA for 20,904 registered adult users of local authority leisure facilities in Northumberland, United Kingdom, between July 2018 and June 2019, using administrative data. The authors categorized activity levels (<30, 30-149, and ≥150 min/wk) and used ordinal regression to examine predictors for activity category achieved. RESULTS: Registered users were mainly female (58.7%), younger (23.9% of users aged 18-29 y vs 10.1% of those aged 70+ y), and from the 2 most affluent socioeconomic quintiles (53.7%). Median weekly moderate/vigorous leisure center-based activity was 55 minutes per week (interquartile range: 30-99). Being female (odds ratio: 2.09; 95% confidence interval, 1.95-2.35), older (odds ratio: 1.14; 95% confidence interval, 1.11-1.16), and using a large facility (odds ratio: 1.21; 95% confidence interval, 1.03-1.42) were positive predictors of leisure center-based PA. CONCLUSION: Older adults and females were more likely to be active and achieve the recommended PA levels through usage of the centers. Widespread use of this novel measure of leisure center-based activity would improve the understanding of how local authority leisure centers can address physical inactivity and its associated inequalities.