Previously, we observed no significant reductions in sitting time of the multicomponent Dynamic Work (DW) intervention among office workers. In this study we used mixed-method data to understand context, implementation (i.e. recruitment and delivery) and mechanism of impact (i.e. experiences) of the DW intervention and to explore whether an higher implementation index score led to larger changes in participants' outcomes. We found considerable variation across departments regarding context (i.e. different size and work tasks) and implementation (i.e. delivery varied). Satisfaction with the DW intervention was high. An higher implementation index score was associated with lower overall sitting time, lower occupational sitting time, higher number of steps/day and steps/day at work at 4-months, which was maintained at 8-month for occupational sitting time. These findings provide an understanding that implementation was affected by a lack of availability of intervention components, department policy, work tasks, positioning and work location. TRIAL REGISTRATION: Clinicaltrials.gov, registration number:NCT03115645. Registered February 17, 2017 - Retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT03115645.