BACKGROUND: While many environmental exposures in childhood have been associated with later risk of mental disorders, less is known about whether cumulative early-life exposure to ambient temperature could be relevant to later mental health. We combined high-resolution meteorological data with individual-level data from Finnish nationwide registers to investigate associations of childhood ambient temperature exposure with subsequent mental disorders. METHODS: The cohort included 578 067 Finnish individuals born in 1990-1999. For each individual, we calculated average daily exposure to ambient temperature from birth to their 10th birthday based on residential history. The individuals were followed from their 10th birthday until a mental disorder diagnosis, death, emigration, or December 2019. Cox proportional hazards models were used to estimate the associations of childhood temperature exposure with subsequent mental disorders while adjusting for individual-level and area-level confounders. FINDINGS: Childhood exposure to warmer climatic conditions was monotonically associated with a greater subsequent risk of a mental disorder (compared to the median of the 10-year temperature exposure, HR = 1.10 (95% CI, 1.08-1.12) at the 95th percentile, HR = 0.88 (95% CI, 0.86-0.90) at the 5th percentile). In disorder-specific analyses, the associations were evident for mood and anxiety disorders, and to a lesser extent for sleep and substance use disorders. No consistent evidence was observed with psychotic or eating disorders. INTERPRETATION: Growing up in warmer climatic conditions was associated with subsequent risk of mental disorders. Further studies in populations residing in different latitudes are needed.