While donor-conceived children have similar mental health outcomes compared to spontaneously conceived children, there is an inconsistency between studies investigating mental health outcomes of donor-conceived people in adulthood. This study is an online health survey that was completed by 272 donor sperm-conceived adults and 877 spontaneously conceived adults from around the world. Donor sperm-conceived adults had increased diagnoses of attention deficit disorder (P = 0.004), and autism (P = 0.044) in comparison to those conceived spontaneously. Donor sperm-conceived adults self-reported increased incidences of seeing a mental health professional (P < 0.001), identity formation problems (P < 0.001), learning difficulties (P < 0.001), panic attacks (P = 0.038), recurrent nightmares (sperm P = 0.038), and alcohol/drug dependency (P = 0.037). DASS-21 analysis revealed that donor sperm-conceived adults were also more stressed than those conceived spontaneously (P = 0.013). Both donor sperm and spontaneously conceived cohorts were matched for sex, age, height, alcohol consumption, smoking, exercise, own fertility, and maternal smoking. The increase in adverse mental health outcomes is consistent with some studies of donor-conceived adult mental health outcomes. These results are also consistent with the Developmental Origins of Health and Disease (DOHaD) phenomenon that has linked adverse perinatal outcomes, which have been observed in donor-conceived neonates, to increased risk of chronic disease, including mental health. Further work is required to reconcile our observations in adults to contrary observations reported in donor-conceived children.