Social prescribing is an innovative approach that aims to improve health and wellbeing by addressing non-medical needs through community-based supports and services. This approach can be delivered through link workers, who work closely with participants to understand their needs and connect them to appropriate services. While social prescribing shows potential for cost savings and improved care quality, further research is needed to understand its effectiveness for people experiencing mental health problems. This pre-post evaluation examined the impact of a social prescribing intervention on health-related quality of life, subjective health, wellbeing, and psychological distress among Australian adults diagnosed with mental health conditions. Adults (aged 18 years and over) from metropolitan Northern Sydney region in New South Wales, who either self-presented to their general practitioner or self-referred were eligible. 398 eligible participants completed the program between December 2021 and August 2024 (data cut-off date) from an ongoing initiative. Participants were assigned to a link worker within 4-6 weeks of initial intake. The 12-week social prescribing program involved link workers co-designing individualised plans addressing specific needs, providing support and revising plans when required. Validated instruments measuring quality of life, wellbeing, and psychological distress were administered pre- and post-intervention, alongside a satisfaction survey. Program enrolment lasted a mean of 17.6±7.6 weeks (median 16.6, range: 1-48). Participants experienced improvements in health-related quality of life (p<0.001), mental wellbeing (p<0.001), general wellbeing (p<0.001), subjective health (p<0.001), and psychological distress (p<0.001). Benefits were consistent across binary gender, clustered mental health diagnoses, and clustered care intensity levels (lower and higher). These findings suggest that social prescribing programmes can improve wellbeing outcomes among people experiencing mental illness, highlighting the potential for broader implementation within Australian mental healthcare systems.