The COVID-19 pandemic has affected the life of children and adolescents in an unpredecented way, limiting, among others, everyday activities with direct social contacts to mitigate the spread of the virus. These limitations have been associated with worse mental health. Yet, little is known about the underlying mechanisms. In the present study, we focused on two activities that have been likely affected by mitigation measures: screen time and green time. We investigated how screen time and green time influenced each during the pandemic, how they affected children’s and adolescents' mental health, and which role socio-demographic characteristics have in predicting screen time, green time, and mental health. We used data collected over between autumn 2020 and spring 2021 from 844 participants aged 5 to 19 of a population-based, prospective cohort study in Ticino, Switzerland. We analyzed the data using an extended version of the Random Intercept Cross-Lagged Panel Model with time-invariant socio-demographic covariates and mental health as outcome. Results showed that, at the between-person level, screen time was a risk factor and green time a protective factor of mental health. However, within-person deviations of screen time and green time during the pandemic did not consistently predict mental health. Furthermore, they did not influence each other over time. Gender, age, socio-economic background, Body Mass Index and the availability of green space nearby all influenced stable measures of green time and screen time (i.e., random intercepts). Our results highlight the need for targeted actions to promote green time and raise awareness about the detrimental effect of screen time on children’s and adolescents’ mental health.