The question of whether mental disorders are brain disorders has sparked curiosity in cognitive science for years. But does framing mental disorders as brain disorders actually help the public better understand and engage with mental health? What do people understand when we call something a brain disorder, and why does it matter if mental illnesses are described as brain-based? To explore these questions, we conducted three quantitative vignette studies with a UK-based general public sample, focusing on perceptions of seven mental disorders: ADHD, ASD, OCD, major depressive disorder, anorexia nervosa, schizophrenia, and bipolar disorder (as defined in the DSM-5-TR). Our findings show that seeing mental disorders as brain disorders is linked to beliefs about greater severity, earlier onset, longer duration, lower chances of recovery, and higher effectiveness of medication. These results highlight how public perceptions might impact reasoning and decision-making about mental health.