Examining associations of mother's behaviors and home/neighborhood physical activity (PA) environments with preschoolers' PA and screen time. Cross-sectional online survey. Mothers with a 2 to 5 years old preschooler were recruited from the US panel members of Survey Sampling International. Five hundred thirty-one mothers with a preschool child aged 2 to 5 years old. Child daily screen time and PA, mother-child inside- and outside-home co-PA. K-mean cluster analysis and Logit and negative binomial regressions. Mothers' healthy behaviors, such as decreased screen time, healthy eating habits, and increased PA, and perceived importance for PA were significantly (P < .05) associated with preschoolers' decreased screen time and increased PA. Available toys (P < .01) and maternal perceived neighborhood safety (P < .05) were negatively correlated with preschoolers' screen time, while available room space (P < .01) was positively correlated with preschoolers' PA. Variables positively correlated with mother-child co-PA included mothers' PA (P < .001) and healthy eating habits (P < .05), and home room space (P < .05) for inside-home, and yard space and quality (P < .05) for outside-home. Mother's role modeling and home PA environment were positively associated with preschoolers' PA behavior.