Introduction: The COVID-19 pandemic has adversely affected health systems in many countries, but little is known about effects on health systems in sub-Saharan Africa. This study examines the effects of COVID-19 on health service utilisation in a sub-Saharan country, Sierra Leone. Methods: Mixed-methods study using longitudinal nationwide hospital data (admissions, operations, deliveries and referrals), and qualitative interviews with healthcare workers and patients. Hospital data were compared across Quarters (Q) in 2020, with day 1 of Q2 representing the start of the pandemic in Sierra Leone. Admissions are reported in total and disaggregated by sex, service (surgical, medical, maternity, paediatric), and hospital type (government or private not for profit). Referrals in 2020 were compared with 2019, to assess whether any changes were the result of seasonality. Comparisons were performed using students t test. Qualitative data were analysed using thematic analysis. Results: From Q1-Q2, weekly mean hospital admissions decreased by 14.7% (p=0.005). Larger decreases were seen in male 18.8%, than female 12.5% admissions. The largest decreases were in surgical admissions, a 49.8 % decrease (p<0.001) and medical admissions, a 28.7% decrease (p=0.002). Paediatric and maternity admissions did not significantly change. Total operations decreased by 13.9% (p<0.001), whilst caesarean sections and facility-based deliveries showed significant increases, 12.7 % (p=0.014) and 7.5% (p=0.03) respectively. In Q3 total admissions remained 13.2% lower (p<0.001) than Q1. Mean weekly referrals were lower in Q2 and Q3 of 2020 compared to 2019, suggesting findings were unlikely to be seasonal. Qualitative analysis identified both supply-side factors, prioritisation of essential services, introduction of COVID-19 services and pausing elective care, and demand-side factors, fear of nosocomial infection and financial hardship. Conclusion: The study demonstrated a decrease in health service utilisation during Covid-19, the decrease is less than in other countries during COVID-19 and less than reported during the Ebola epidemic.