Objective Smoking is the largest preventable cause of death and disability in the UK and remains pervasive in people with mental disorders and in general hospital patients. We aimed to quantify the prevalence of mental disorders and smoking, examining associations between mental disorders and smoking in patients with chronic physical conditions. Method Data were collected via routine screening systems implemented across two London NHS Foundation Trusts. The prevalence of mental disorder, current smoking, nicotine dependence and wanting help with quitting smoking were quantified, and the relationships between mental disorder and smoking were examined, adjusting for age, gender and physical illness, with multiple regression models. Results A total of 7878 patients were screened; 23.2% screened positive for probable major depressive disorder, and 18.5% for probable generalised anxiety disorder. Overall, 31.4% and 29.2% of patients with probable major depressive disorder or generalised anxiety disorder respectively were current smokers. Probable major depression and generalised anxiety disorder were associated with 93% and 44% increased odds of being a current smoker respectively. Patients with depressive disorder also reported higher levels of nicotine dependence, and the presence of common mental disorder was not associated with odds of wanting help with quitting smoking. Conclusion Common mental disorder in patients with chronic physical health conditions is a risk factor for markedly increased smoking prevalence and nicotine dependence. A general hospital encounter represents an opportunity to help patients who may benefit from such interventions.