This mixed-method study explores how extreme heat affects mental health, combining a systematic review and focus groups. The review identified limited evidence for mechanisms like HPA axis activation, stress, sleep disruption, and reduced physical activity. Focus groups with people with lived experience and healthcare professionals added insights into fatigue, anxiety, coping challenges, and social isolation. The study highlights the lack of robust data and the need to center lived experience in future research. Understanding these mechanisms is key to developing effective interventions and mental health strategies in a warming world.