The mental health effects of weather-related disasters are well characterized, yet less is known about the effect of chronic, slow-onset climate change. We systematically reviewed qualitative, quantitative and mixed-methods studies (57 were included) that investigated the effects of slow-onset climate change on a range of mental health indicators. Droughts, changing temperatures over time and local perceptions of ecosystem changes were the most studied slow-onset conditions. Several quantitative studies noted adverse mental health outcomes associated with these exposures, including depression and anxiety symptoms, suicide and non-specific psychological distress. Qualitative studies further elucidated negative emotions related to chronic climate change, including worry, grief and frustration. However, some studies noted mixed or null findings. Results suggest a need for further research to identify causal pathways and mechanisms through which chronic changes in the climate may affect changes in mental health. Instead of focusing on trauma-based frameworks (as are commonly used in studies of acute disasters), this work should holistically consider individual, community and societal factors that shape the mental health consequences of slow-onset climate change.