Improving mental wellbeing has a range of benefits for society, including increased productivity, longevity, and resiliency. However, interventions designed to improve mental wellbeing are often only compared to waitlist controls, leaving uncertainty regarding the mechanisms of their effectiveness. The current study in 326 participants assessed a six-week positive psychology intervention against an active control (n = 163) in an online randomized control trial. Outcome measures included life satisfaction, wellbeing (subjective and psychological wellbeing), stress, depression and anxiety symptoms, and self-compassion. The potential moderating effect of participating during the ongoing COVID-19 pandemic was also explored. The intervention group showed greater improvements in life satisfaction by week six (beta = 0.18, p = .014) and were maintained through to 7 weeks post-baseline (beta = 0.23, t = 3.07, p = .002) and remained significant when accounting for COVID-19 restrictions. An improvement in composite wellbeing from baseline to 7 weeks post-baseline was detected when accounting for COVID-19 restrictions. Composite wellbeing and total depression and anxiety symptoms improved significantly more in the intervention group for participants with low baseline resiliency resources. These findings support the efficacy of using online multi-component positive psychology interventions in boosting wellbeing and reducing distress symptoms particularly in individuals with fewer resiliency resources who may need added support.