Background: To investigate whether light exposure was associated with objectively measured physical activity (PA) and sedentary behaviour in young people. Methods: Participants (n = 229, 46.7% female) were young people (mean 8.8 years [SD ± 2.2]) from the borough of Camden, UK. Daily sedentary time, moderate and vigorous PA (MVPA) and light exposure were measured using a tri-axial accelerometer with an ambient light sensor during the summer. Multiple linear regression models examined associations between average daily light exposure, sedentary time and time in MVPA. Models were repeated investigating weekdays and weekend days separately. Analyses were adjusted for pre-specified covariables, including age, sex, device wear time, ethnic group, school and body fat. Results: There were significant associations between average daily light exposure and time sedentary (β coefficient = −11.2, 95% CI, −19.0 to −3.4) and in MVPA (β coefficient = 3.5, 95% CI, 1.2 to 5.9). Light exposure was significantly associated with weekend sedentary time (β coefficient = −10.0, 95% CI, −17.6, −2.4), weekend MVPA (β coefficient = 3.7, 95% CI, 1.7, 5.7), weekday sedentary time (β coefficient = −15.0, 95% CI, −22.7 to −7.2), but not weekday MVPA (β coefficient = 2.0, 95% CI, −0.5 to 4.5). Conclusion: Average daily light exposure is positively associated with time in MVPA and negatively associated with sedentary time. Increasing daylight exposure may be a useful intervention strategy for promoting physical activity.