Introduction The emergence of COVID-19 and the importance of behaviour change to limit its spread created an urgent need to apply behavioural science to public health. Knowledge mobilisation, the processes whereby research leads to useful findings that are implemented to affect positive outcomes, is a goal for researchers, policy makers and practitioners alike. This study aimed to explores the experience of using behavioural science in public health during COVID-19, to discover barriers and facilitators and whether the rapidly changing context of COVID-19 influenced knowledge mobilisation. Methods We conducted a semi-structured interview study, with ten behavioural scientists and seven public health professionals in England, Scotland, Wales, The Netherlands and Canada. We conducted an inductive thematic analysis. Results We report three key themes and 10 sub-themes: 1.Challenges and facilitators of translation of behavioural science into public health (Methods and frameworks supported translation, Lack of supportive infrastructure, Conviction and sourcing of evidence and Embracing behavioural science) 2. The unique context of translation (Rapid change in context, the multi-disciplinary team and the emotional toll). 3. Recommendations to support future behavioural science translation (Embedding experts into teams, Importance of a collaborative network and showcasing the role of behavioural science). Discussion Barriers and facilitators included factors related to relationships between people, such as networks and teams; the expertise of individual people; and those related to materials, such as the use of frameworks and an overwhelming amount of evidence and literature. Conclusion People and frameworks were seen as important in facilitating behavioural science in practice. Future research could explore how different frameworks are used. We recommend a stepped competency framework for behavioural science in public health and more focus on nurturing networks to facilitate knowledge mobilisation in future emergencies.