Postpartum weight retention is a major contributor to obesity in later life resulting in long-term health consequences in women. Postpartum lifestyle interventions are known to be effective in reducing postpartum weight retention and improving the overall health and wellbeing of mothers but have poor reach and engagement. This study describes the engagement of mothers with young children in the development of a theory- and evidence-based intervention to reduce postpartum weight retention. A participatory design methodology with input from a community mothers'group, literature reviewsand an expert advisory group was applied. Mothers who were members of 'Mothers of Preschoolers' (MOPS) were invited to participate in a focus group discussion and two co-design workshop sessions. Thirteen women participated in a focus group discussion and 12 women in each co-design workshop. We found that mothers valued having social support from their peers, practical support such as meal delivery, and learning opportunities that focus on the mother's health and wellbeing. The advisory group suggested leveraging the unique skills and prior experiences of mothers within the group and developing a curriculum that mothers can be trained to deliver. A program that emphasizes the strengths and value of mothers can increase their self-worth and self-confidence resulting in intrinsic motivation to improve lifestyle behaviours. An intervention designed to be implemented by MOPS for its members and incorporated into their regular sessions has the potential for feasibility and acceptability among mothers with young children. Mothers with young children were part of the program planners and were involved in the design and conduct of this study and in the interpretation of the findings. A member of a community mothers'group recruited other mothers with young children within the group to participate in a series of sessions to discuss their experiences of the postpartum period and preferences for a lifestyle program. The mothers identified the behavioural outcomes and program goals for a postpartum lifestyle program and then generated the program ideas based on these.