The increase in overweight and obesity among women is a growing concern, and reproduction is associated with persistent weight gain. We have shown that dietary behavioural modification treatment, with or without exercise, results in weight loss and maintenance of weight loss. The aim of this study was to provide an explanatory model of how overweight and obese women achieve weight loss during, and after, participating in a post-partum diet and/or exercise intervention. Using Grounded Theory, we performed and analysed 29 interviews with 21 women in a 12-week Swedish post-partum lifestyle intervention with a 9-month follow-up. Interviews were made after the intervention and at the 9-month follow-up. To overcome initial barriers to weight loss, the women needed a ‘Catalytic Interaction’ (CI) from the care provider. It depended on individualised, concrete, specific and useful information, and an emotional bond through joint commitment, trust and accountability. Weight loss was underpinned by gradual introduction of conventional health behaviours. However, the implementation depended on the experience of the core category process ‘Transformative Lifestyle Change’ (TLC). This developed through a transformative process of reciprocal changes in cognitions, emotions, body, environment, behaviours and perceived self. Women accomplishing the stages of the TLC process were successful in weight loss, in contrast to those who did not. The TLC process, dependent on initiation through CI, led to implementation and integration of recognised health behaviours, resulting in sustainable weight loss. The TLC model, including the CI construct and definition of barriers, facilitators and strategies provides an explanatory model of this process.