COVID-19 restrictions could potentially induce poor mental health. This study considers opposing government restrictions on outdoor physical activity in Italy and the UK to evaluate participants' ability to conduct physical activity, the relationship between physical activity and mental health, and whether restrictions affect mental health as mediated by physical activity. Participants from the UK and Italy self-reported physical activity before and during restrictions, sociodemographic data, and symptoms of depressions, stress, and anxiety during restrictions. Changes in physical activity were analyzed by tests of differences, and relationships between mental health, physical activity, and the effect of country restrictions were evaluated through path analysis. Two hundred sixty-four participants (UK: 57%; Italy: 43%) responded. The model (χ2(2) = .429,  > .05; RMSEA < .08; CFI > .90) confirmed the country's effect on physical activity. Participants from Italy significantly decreased physical activity while the least active individuals in the UK increased activity during restrictions. Decreases in physical activity had a significant effect on increased reports of stress and depression. Physical activity did not mediate mental health within the countries. Future policies may consider ways to support individuals in maintaining physical activity habits to minimize the possibility of worsening mental health.