OBJECTIVES: We aimed to assess mobility patterns and reasons for high mobility among young women engaged in sex work within a randomised controlled trial to gauge how mobility may hinder access to health services and enhance HIV risk in a highly vulnerable population. SETTING: Participants were recruited from a clinic in Kampala, Uganda set up for women at high risk of HIV infection. PARTICIPANTS: Adolescent girls and young women engaged in sex for money and/or commodities are at particular risk in countries with high HIV prevalence and high fertility rates. High mobility increases exposure to HIV risk. Women participants were eligible for the parent study if aged 15-24 years, HIV negative and engaged in sex work. For this substudy, 34 qualitative interviews were held with 14 sex workers (6 HIV positive, 8 HIV negative), 6 health worker/policy makers, 3 peer educators, 5 'queen mothers' and 6 male partners MEASURES: Participants used Google Maps to identify work venues at 12-month and 18-month study visits. We also conducted 34 interviews on mobility with: high-risk women, male partners, health workers and sex-worker managers. Topics included: distance, frequency and reasons for mobility. We used Python software to analyse mapping data. RESULTS: Interviews found in depth narratives describing lack of education and employment opportunities, violence, lack of agency, social, sexual and familial support networks and poverty as a complex web of reasons for high mobility among young sex workers. CONCLUSIONS: Young women at high risk are highly mobile. Reasons for mobility impact access and retention to health services and research activities. Strategies to improve retention in care should be cognisant and tailored to suit mobility patterns. TRIAL REGISTRATION NUMBER: NCT03203200.