Mental illness as a failure to make necessary satisfactory adaptation to the demand of the society: this study intends to investigate into the prevalence of mental illness among adult. A case study of Ilesha East Local Government Area, State of Osun. The main aim and objective is to identify the main causes of mental illness among adult in the selected area. The population of the study includes one hundred respondents. A descriptive research work and simple random sampling and sample of convenience were adopted for this study. Questionnaire was used for the collection of data; frequency distribution table was used to analyze it. From findings the respondent has moderate knowledge on mental illness and the major causes of mental illness among adult in the study area was done due to excessive consumption of alcohol followed by smoking of Indian hemps. Based on findings, the incidence of consumption of alcohol and drug abuse is still high in the selected study area.