There is a strong body of evidence that points to the mental health and well-being benefits of nature alongside a growing evidence based on the health and well-being benefits of green social prescribing. Central to the UK government’s commitment to transform mental health services, seven successful ‘test and learn’ green social prescribing sites were identified across England, including the Humber and North Yorkshire green social prescribing programme. We used a cohort design to compare participants’ mental health and well-being outcomes before and after exposure to nature-based interventions offered via a referral from social prescribing organisations linked to the green social prescribing initiative. Data were collected between February 2022 and March 2023 using before and after questionnaires to determine the impact of green social prescribing on mental health and well-being outcomes using ONS-4 and Hospital and Anxiety Depression scales. In total, two hundred and twenty-three participants from across Humber and North Yorkshire were included in the cohort evaluation. Survey data were analysed quantitatively using descriptive and multivariable analyses. After adjustments for deprivation and health status, larger benefits for improvements in well-being were found to be associated with nature-based activities with durations of nine to 12 weeks compared to those with durations of one to four weeks. Horticultural and care farming activities were also shown to be associated with some improvements in well-being over nature-based sports and exercise activities. We have undertaken the first longitudinal and quantitative evaluation of the NHS England green social prescribing “test and learn” programme across Humber and North Yorkshire. Our evaluation in Humber and North Yorkshire showed green social prescribing is beneficial for mental health and well-being among community-based adults, suggesting potential as a timely and relevant community-based intervention to address the demand for mental health care in an era of high unmet need.