This study examines how Sri Lanka, a lower-middle income country, managed its COVID-19 response and maintained health services. It draws on an extensive document review, key informant interviews and a national survey of public experience and opinion to assess what Sri Lanka did, its effectiveness and why.Owing to a strong health system and luck, Sri Lanka stopped the first wave of COVID-19 infections, and it adopted a 'Zero-COVID' approach with the explicit goal of stopping outbreaks. This was initially effective. Outbreaks reduced healthcare use, but with minimal impact on health outcomes. But from end-2020, Sri Lanka switched its approach to tolerating transmission and mitigation. It took proactive actions to maintain healthcare access, and it pursued a COVID-19 vaccination effort that was successful in covering its adult population rapidly and with minimal disparities. Despite this, widespread transmission during 2021-2022 disrupted health services through the pressure on health facilities of patients with COVID-19 and infection of healthcare workers, and because COVID-19 anxiety discouraged patients from seeking healthcare. This led to substantial mortality and more than 30 000 excess deaths by 2022.We find that Sri Lanka abandoned its initially successful approach, because it failed to understand that its chosen strategy required symptomatic PCR testing in primary care. Failure to invest in testing was compounded by groupthink and a medical culture averse to testing.Sri Lanka's experience confirms that strong public health capacities, robust healthcare systems and intersectoral action are critical for pandemic response. It shows that civilian-military collaboration can be beneficial but contested, and that lack of fiscal space will undermine any response. It also demonstrates that pandemic preparedness cannot guarantee a successful pandemic response. Policy and research must pay more attention to improving decision-making processes when faced with pandemics involving novel pathogens, rapid spread, and substantial scientific uncertainty.