ABSTRACT Objectives: Farmers are faced with many stressors, along with numerous barriers to accessing traditional forms of mental health support. The ifarmwell online intervention was co-designed with farmers and is informed by Acceptance and Commitment Therapy to equip farmers with transferable coping strategies, particularly to help them cope with circumstances beyond their control. We aimed to evaluate the effect of ifarmwell on farmers’ short- and long-term distress and mental wellbeing. Methods: Australian farmers (21–73 years) who registered on www.ifarmwell.com.au completed measures at the commencement of module 1 (N = 228), the end of the intervention (N = 77) and 6-months post-intervention (N = 61). Primary outcomes included distress (Kessler Psychological Distress Scale) and mental wellbeing (Mental Health Continuum – Short Form). We also examined the relationship between distress, mental wellbeing, neuroticism (Quickscales-R) and ACT-based psychological mechanisms; psychological inflexibility (Acceptance and Action Questionnaire-II), cognitive fusion (Cognitive Fusion Questionnaire), believability of automatic thoughts (Automatic Thoughts Questionnaire-B), coping via acceptance (4 items from the situational COPE) and mindfulness (Five Facet Mindfulness Questionnaire-Short Form). Secondary outcomes of acceptability (Client Satisfaction Questionnaire-8) and usability (System Usability Scale) were also explored. Results: Pre- to post-intervention, farmers’ distress decreased and mental wellbeing increased. These effects were maintained at 6-month follow-up. Changes in these outcomes were greatest for participants who entered the intervention with high baseline distress (= −0.59, 95%CI =[−0.70, −0.47]) and low mental wellbeing (= −0.33, 95%CI = [−0.47, −0.19]). Decreases in distress and increases in mental wellbeing were associated with decreases in psychological inflexibility, cognitive fusion, and believability of thoughts, and increases in coping via acceptance and mindfulness. Levels of satisfaction (M = 26.92/32) and usability (M = 84.70/100) were high, and 94.6% of participants said they would recommend ifarmwell to a friend in need of similar advice and tools. Conclusions: ifarmwell is an effective and usable intervention that is likely to help farmers reduce their levels of distress and improve their mental wellbeing, by improving their psychological flexibility, ability to focus on the present and accept things beyond their control, as well as by reducing the extent to which they believe unhelpful thoughts.