The UK’s decision to leave the EU has ramifications beyond the UK and the EU. This article analyses the impact of the Brexit referendum on the UK’s political capital in the UN Security Council; a dimension of Brexit which has received little attention thus far. Drawing on extensive elite interviews we show that the UK has considerable political capital in the Council where it is seen as one of the most effective actors, however the reputational costs of Brexit are tarnishing this image. With case studies on the UK’s role in Somalia and Yemen we show how the UK has been able to further its interests with dual roles in the EU and Security Council, and the risks posed by tensions between trade and human rights after Brexit. We also analyze what it takes to be influential within the Security Council and argue that more attention should be paid to the process of diplomacy. Influence is gained via penholding, strong diplomatic skill, and a well-regarded UN permanent representative. The UK accrues political capital as a leader on the humanitarian and human rights side of the Council’s agenda, but this reputation is at risk as it exits the EU.