Background: Recent evidence suggests that multiple symptoms or diagnoses, partucularly when co-ocuring with non-suicidal self-harm, predict suicide risk more strongly than single diagnosis. Method: Suicidal thought (ST) and non-suicidal self-injury (NSSI) were studies in two independent longitudinal UK samples of young people: the Neuroscience in Psychiatry (NSPN) 2400 cohort (n=2403) and the ROOTS cohort (n=1074). Participants, age 14-24 years, were recruited from primary health care registers, schools and colleges, and advertisements to complete quotas in age-sex strata. We calculated a score on a latent construct Common Mental Distress (the summary measure indexing a broad range of symptoms conventionally seen as components of distinct disorders). We examined the relative prevalence of ST and NSSI over the population distribution of mental distress; we used logistic regressions, IRT and ROC analyses to determine associations between suicide risks and mental distress (in continuous and above-the-norm categorical format); and pathway mediation models to examine longitudinal associations. Outcomes: We found a dose-response relationship between levels of mental distress and suicide risk. In both cohorts the majority of all subjects experiencing ST (78% and 76%) and NSSI (66% and 71%) had scores on mental distress no more than two standard deviations above the population mean; higher scores indicated highest risk but were, by definition, infrequent. Mental distress contributed to the longitudinal persistence of ST and NSSI. Interpretation: Universal prevention strategies reducing levels of mental distress in the whole population (in addition to screening) may prevent more suicides than approaches targeting youths with psychiatric disorders. Funding Statement: The ROOTS study was supported by a Wellcome Trust Grant (Grant number 074296) to I.M.G. and P.B.J., the NIHR Collaborations for Leadership in Applied Research and Care (CLAHRC) East of England, and the NIHR Cambridge Biomedical Research Centre. The NSPN study was supported by the Wellcome Trust Strategic Award (095844/Z/11/Z) to I.M.G., E.B., P.B.J., R.D., P.F. The work has been carried out in the Department of Psychiatry, University of Cambridge. Declaration of Interests: E.P., S.N., I.M.G., J.S. and P.B.J. have no competing interests. E.B., P.F. and PBJ are in receipt of National Institute for Health Research (NIHR) Senior Investigator Awards (NF-SI0514-10157, and NF-SI-0514-10117. P.F. was in part supported by the NIHR Collaboration for Leadership in Applied Health Research and Care (CLAHRC) North Thames at Barts Health NHS Trust. P.W. has recent/current grant support from NIHR, Cambridgeshire County Council and CLAHRC East of England. P.W. discloses consulting for Lundbeck and Takeda; PBJ discloses consulting for Janssen and Ricordati. E.B. is employed half-time by the University of Cambridge and half-time by GlaxoSmithKline in which he holds stock. Ethics Approval Statement: Written consent from participants age 14 or 15 years was supplemented by written consent from their parent or legal guardian; older participants gave their own written consent. Ethical approval was obtained for Cohort 1 from the National Health Service Research Ethics Service (# 97546) and for Cohort 2 from the Cambridgeshire 2 REC (# 03/302).