On the basis of information collected from all the published European Commission's decisions in cartel cases between 1962 and 2014, this paper identifies different stages in the supra‐nationalization of cartel policy at the European Union (EU) level. It analyzes major competition policy reforms, strategies and initiatives taken by the European Commission (EC), and its relationship and interaction with Member States' resistance and other policy players' positions, which offers a new in‐depth study on the history and political economy of a key pillar of EU integration. It also provides a forensic analysis of the sanctioned cartels at each stage. This study shows that the introduction of the leniency programme was a critical juncture that allowed cartel authorities to identify cartels more effectively and to provide evidence for sanctioning collusion much more easily than before. This success was a key determinant for deepening the EU integration in competition policy. The intended drivers and the paradoxically unexpected shifters of such growing integration in cartel policy enforcement at the EU are discussed.