Objectives: This study investigated the impact of a peer support intervention on the breakfast habits of university students in Japan. Both breakfast frequency and breakfast composition were assessed. Design: The study adopted a combination of convergent and intervention designs and used a mixed-methods approach. Setting: College-based peer-support intervention taking place in Sapporo, Hokkaido, Japan. Method: Questionnaires on demographic characteristics and breakfast habits were administered to participants (n = 27) before the online workshop, immediately afterwards, and at 3 and 6 months after the peer support intervention, coupled with a 4-week breakfast status report. Semi-structured interviews were then conducted with 17 participants to gain a more detailed understanding of the impact of the intervention. Results: Quantitative results showed that immediately after the intervention, breakfast intake frequency and breakfast composition improved significantly; however, post-intervention, there was a difference in persistence between frequency and composition. In the qualitative element of the study, 'peer stimulation' and 'coercion' were identified as drivers of the changes identified. In addition, 'changes in awareness and behaviour' and 'realisation of the need for breakfast' were noted by participants. Conclusion: Peer support interventions can improve the breakfast habits of university students. A follow-up intervention within 6 months for breakfast frequency and within 3 months for breakfast composition following the intervention may help sustain the observed beneficial effects.