Increasing numbers of people are employed in sedentary occupations, spending large amounts of time sitting at work which is detrimental to health and wellbeing. Evidence-based guidance is required to intervene to reduce sedentary behaviour, encourage physical activity and promote sustainable working. This article presents a process evaluation of a successful workplace intervention Walking Works Wonders, shown to be effective in improving health, job satisfaction and motivation (Haslam et al. 2018). In this qualitative process evaluation employees reported an increased awareness of their sedentary time and they particularly valued the monitoring of activity using pedometers. They described changes to their working and leisure time activity to accumulate more steps. Participants reported improved physiological and psychological health outcomes, improved working relations with colleagues, changes in dietary behaviour and involving their families in physical activity. The results highlight elements of the intervention that encouraged healthy and more sustainable working practices.Practitioner summary: This study provides the employees' perspective on the effective elements of a workplace intervention which encouraged physical activity and reduced sitting time. The results offer valuable insights for practitioners aiming to develop interventions to improve health and facilitate more sustainable working practices in a rapidly changing world of work.