Tobacco use remains the leading cause of preventable death worldwide. Though research on smoke-free policies in the U.S. exists at the state or national level, there is limited evidence on such policies at the county level. This study examined the association between changes in county-level comprehensive smoke-free laws and smoking behavior among U.S. adults.Data were used from the 1995/1996 to the 2014/2015 Tobacco Use Supplement to the Current Population Survey, analyzed in 2019. Changes in county smoke-free law population coverage over time were used as a natural experiment. Quasi-experimental analyses were performed to examine the association between changes in county-level comprehensive smoke-free laws and smoking behavior (smoking status and number of cigarettes smoked).During the study period, current smoking prevalence decreased from 21.6% to 11.9%. Specifically, the prevalence of every day smoking decreased from 17.1% to 9.1% and some days smoking decreased from 4.4% to 2.9%. The number of cigarettes smoked per day for every day smokers decreased from 18.5 to 13.6, and from 5.9 to 4.1 for those who smoked some days. Comprehensive smoke-free legislation was associated with lower odds of cigarette smoking (AOR=0.76, 95% CI=0.74, 0.79). Adults living in counties with comprehensive smoke-free policies smoked fewer cigarettes per day both for every day smokers (β= -1.55, p<0.0001) and some days smokers (β= -0.79, p<0.0001).County smoke-free policies in the U.S. may have contributed significantly to the reduction in smoking prevalence as well as the reduction in the number of cigarettes smoked among continuing cigarette smokers.