The cyclical nature of agriculture exposes farmers to challenges that vary over time, influenced by a myriad of external factors beyond their control. This longitudinal study aimed to examine mental health trajectories for Western Australian farmers over a 12 month period, and determine associations with key risk and protective factors. Farmers and farm residents (N = 125) completed an online survey at three timepoints (baseline, 6 months and 12 months) that assessed anxiety, depression, stress, farming stressors, coping strategies, sense of belonging and social supports. Survey timing coincided with periods of harvest and seeding. Risk factors associated with poorer mental health among farmers included a range of dysfunctional and emotion coping strategies, family/relationship stressors, financial/external stressors and daily stressors. Protective factors included sense of belonging and social support. Stress was found to change over time and family/relationship stressors and financial/external stressors at baseline were found to moderate anxiety and stress at 6 months and 12 months. Addressing factors associated with poor mental health trajectories and bolstering factors that are protective for mental health over time will aid clinical and community services in tailoring their services to meet the needs of farmers.