This study examined temporal relationships between relative onsets of mental illness and homelessness in a cross-sectional study of 900 homeless people compared with a matched, never-homeless sample from the Epidemiologic Catchment Area study. All psychiatric disorders preceded homelessness in the majority. Only one disorder, alcohol use disorder (in men only), had significantly earlier onset in homeless subjects. Regarding number of symptoms or earlier age of onset of psychiatric disorders, earlier onset of homelessness was associated with several diagnoses: schizophrenia, major depression, generalized anxiety disorder, alcohol and drug use disorders, and antisocial personality. In multiple regression models, history of dysfunctional family background and maternal psychiatric illness were also associated with earlier onset of homelessness, whereas education was protective. Chronicity of homelessness was associated with number of symptoms of alcohol use disorder and earlier age of onset of drug use disorder, presence and number of symptoms of schizophrenia and antisocial personality, and earlier onset of major depression and conduct disorder. In multiple regression models, more education, but not family background problems, was associated with shorter lifetime duration of homelessness. These findings provide information relevant to the roles of mental illness and personal vulnerability factors in the onset and chronicity of homelessness.