Objectives: To examine the effect that physical activity delivered via two different versions of the Green Prescription (a primary care physical activity scripting program) had on depressive symptomatology and general mental health functioning over a 12-month period in non-depressed, low-active, community-dwelling older adults. Method: Two hundred and twenty-five participants from the Healthy Steps study took part in the present study. Healthy Steps participants were randomized to receive either the standard time-based or a modified pedometer-based Green Prescription. Depression, mental health functioning and physical activity were measured at baseline, post-intervention (3 months post-baseline) and at the 9-month follow-up period. Results: At post-intervention, a positive association was found between increases in leisure-time physical activity and total walking physical activity and a decrease in depressive symptomatology (within the non-depressed range of the GDS-15) and an increase in perceived mental health functioning, regardless of intervention allocation. These improvements were also evident at the follow-up period for participants in both intervention allocation groups. Conclusion: Our findings suggest that the standard time-based Green Prescription and a modified pedometer-based Green Prescription are both effective in maintaining and improving mental health in non-depressed, previously low-active older adults.