Background: We aimed to characterize relationships between delayed high school start time policy, which is known to lengthen school night sleep duration, and patterns in activity outcomes: physical activity, non-school electronic screen time (non-schoolwork), and sports and extracurricular activity among adolescents. Methods: We used data from the START study, a multi-site evaluation of a natural experiment, assessing the effects of a school start time policy change in high schools in the Minneapolis, Minnesota metropolitan area. The study follows students in 2 schools that shifted to a later start time (8:20 or 8:50 am) after baseline year and 3 schools that maintained a consistent, early start time (7:30 am) over the 3-year study period. Activity was measured by participant self-report on an in-school survey. The analysis used a difference-in-differences estimator, in which the changes in each outcome observed in the comparison schools estimate the changes in each outcome that would have been observed in the late-start adopting schools had they not delayed their start times after baseline. Results: Over 2 years of follow-up, no changes emerged to suggest that later school start times either interfered with, or promoted, any activity-related outcome that was measured. Implications: Communities interested in promoting sleep by delaying start times may do so knowing that there are unlikely to be adverse effects on adolescent physical activity, electronic screen time, or organized sports and activity participation. Conclusions: A shift to later school start times does not appear to enhance or detract from the healthfulness of students' activity level.