There is a growing interest in studying and unpacking implementation of policies and programmes as it provides an opportunity to reduce the policy translation time lag taken for research findings to translate to policies and get implemented and understand why policies may fail. Realist evaluation is a theory-driven approach that embraces complexity and helps to identify the mechanisms generating the observed policy outcomes in given context. We aimed to study facilitators and barriers while implementing the Cigarettes and Other Tobacco Products Act, 2003 (COTPA) a comprehensive national tobacco control policy, and the National Tobacco Control Programme (NTCP), 2008 using realist evaluation. We developed an initial program theory (IPT) based on a realist literature review of tobacco control policies in Low- and Middle-Income Countries (LMICs). Three diverse states —Kerala, West Bengal, and Arunachal Pradesh— with varying degree of implementation of tobacco control law and program were chosen as case studies. Within the three selected states, we conducted in-depth interviews with 48 state and district-level stakeholders and undertook non-participant observations to refine the IPT. Following this, we organized two regional consultations covering stakeholders from 20 Indian states for a second iteration to further refine the program theory. A total of 300 Intervention-Context-Actor-Mechanism-Outcome (ICAMO) configurations were developed from the interview data, which were later synthesized into state-specific narrative program theories for Kerala, West Bengal and Arunachal Pradesh. We identified five mechanisms: collective action, felt accountability, individual motivation, fear, and prioritization that were (or were not) triggered leading to diverse implementation outcomes. We identified facilitators and barriers to implementing the COTPA and the NTCP, which have important research and practical implications for furthering the implementation of these policies as well as implementation research in India. In the future, researchers could build on the refined program theory proposed in this study to develop a middle-range theory to explain tobacco control policy implementation in India and other LMICs.