This article explores the relationship between public (i.e. government-led) and private (i.e. industry-led) forms of lobby regulation, by inquiring over the conditions that render a mix between these two regulatory forms more likely. Drawing on the governance of professions scholarship, it presents a theoretically informed comparison between two jurisdictions—the US (federal level) and the EU (supranational level)—where the public regulation of lobbying is built on different models, namely a command-and-control model based on legally binding rules in the US and a cooperative model based on incentives and rewards in the EU. Findings show that, although formal public–private regulatory mixes are not present in either jurisdiction, the EU regulators have hewn closer to this arrangement by coopting lobbyists’ professional associations as semi-permanent but unofficial governance ‘allies’. This outcome is due to an amalgam of factors, chief among which is the severity of political scandals involving lobbyists, which affects policymakers’ reputational incentives for framing the public regulation of lobbying as a punitive or cooperative intervention. Methodologically, the article uses a combination of documentary analysis and expert interviews with lobby practitioners and public regulators on both sides of the Atlantic.