OBJECTIVES: English local authorities (LAs) are interested in reducing alcohol-related harms and may use discretionary powers such as the Late Night Levy (LNL) to do so. This study aims to describe how system stakeholders hypothesise the levy may generate changes and to explore how the system, its actors and the intervention adapt and co-evolve over time. DESIGN: A process evaluation from a complex systems perspective, using qualitative methods. SETTING: A London LA with high densities of residential and commercial properties, which implemented the LNL in 2014. PARTICIPANTS: Data were generated through interviews with LNL implementers and alcohol consumers, observations in bars and during LNL patrols and documentary review. INTERVENTION: The LNL allows LAs to charge late-night alcohol retailers an annual fee (£299-£4440) to manage and police the night-time economy (NTE). RESULTS: When the LNL was being considered, stakeholders from different interest groups advanced diverse opinions about its likely impacts while rarely referencing supporting research evidence. Proponents of the levy argued it could reduce crime and anti-social behaviour by providing additional funds to police and manage the NTE. Critics of the levy hypothesised adverse consequences linked to claims that the intervention would force venues to vary their hours or close, cluster closing times, reduce NTE diversity and undermine public-private partnerships. In the first 2 years, levy-funded patrols developed relationships with the licensed trade and the public. The LNL did not undermine public-private partnerships and while some premises varied their hours, these changes did not undermine the intervention's viability, nor significantly cluster venue closing times, nor obviously damage the area's reputation for having a diverse NTE. CONCLUSIONS: This study applies a framework for process evaluation from a complex systems perspective. The evaluation could be extended to measure alcohol-related outcomes and to consider the interplay between the national and local systems.