Introduction: Mental disorders contributed to 4•7% of DALYs in India in 2017. Institute of Mental Health, (IMH), Chennai, in Tamil Nādu state, India, offers mental health services. We conducted a study to understand patterns of illness and health-seeking behaviour among outpatients in IMH. Methods: We did a cross-sectional study between January to March 2022. We interviewed for socio-demographic characteristics, symptoms, and health-seeking behaviour using a semi-structured questionnaire. We computed proportions for patterns of mental illness and health-seeking behaviour. Results: We enrolled 808 patients. The mean age for males was 34 years and 38 years for females. The majority of patients had higher secondary level education, {males 42% (n= 227) and females 36% (n= 93)}. Median income was Rs.8000. Mental illnesses present were schizophrenia {33% (n=268)}, substance abuse {25% (n=204)}, and bipolar disorder {13%(n=107)}. Reasons for seeking treatment were insomnia {88% (n=714)}, aggression {69% (n=557)}, and unmanageable behaviour at home 61% (n=495). Most patients (70%) sought treatment within 6 months after the onset of symptoms (n=564). For initial treatment, 64% went to the government sector and 36% went to the private sector. More than 90% of the patients had visited psychiatrists working either in government {61%(n=493)} or private hospitals{33%(n=270)}. Only 6 patients (0.7%) had visited traditional healers for initial treatment. Comorbidities present were among 29% (n=231). Comorbidities present were hypertension 8% (n=65), Diabetes 8% (n=62), and seizure 5% (n=39). The prevalence of stigma for mental illness was 81% (n=656). Reasons told by patients for seeking treatment in IMH were satisfactory treatment 66% (n=533) and caregivers were not necessary to stay along30% (n=245). Conclusion: Schizophrenia, substance abuse and bipolar disorders were major illnesses among patients. Health-seeking behaviour was adequate.