Coronary artery disease (CAD) patients are often advised to adhere to healthier lifestyle changes. Previous studies have highlighted the importance of illness perception in various health behaviours of CAD patients, including adherence behaviour. However, it is least explored in the context of Indian patients. The aim of this study is twofold: (i) to explore Indian CAD patients’ illness perceptions and investigate their role in adherence behaviour and (ii) to identify barriers to effective adherence behaviour among them. A cross-sectional mixed methods study involved surveying 236 CAD patients and 22 in-depth interviews were conducted using convenience sampling. Participants completed Illness Perception Questionnaire-Revised (IPQ-R) and adherence behaviour scale developed by the authors. Regression analysis showed that illness perception could significantly predict change in adherence behaviour (Adj. R2 = .21, F = 66.35, p < .00). It was found that treatment control was the only dimension that could significantly predict all four types of studied adherence behaviour. Thematic analysis of the interviews revealed significant barriers for all four studied adherence behaviour, namely, medication adherence, diet adherence, addiction restriction, and adherence to physical activity. This mixed methods study highlights unhealthy illness perceptions and barriers to adherence behaviour, which might be relevant to identify for designing an effective rehabilitation programme for such patients.