Intelligence services have played pivotal and distinct roles in European political history since the Second World War, ranging from their position as a stronghold of authoritarian regimes to their role in protecting democracy from anti-democratic forces. Democratisation therefore requires the depoliticisation of intelligence services to prevent their reuse as tools of political oppression. Paradoxically, many examples show that the politicisation of intelligence often occurs in a context of democratisation. This article explains this paradox. It focuses on the persistent politicisation of the Greek intelligence service in the decades following the demise of the junta regime and its democratic consolidation (1974–2008). It shows that subsequent governments fought fire with fire in their efforts to democratise the service: they countered the junta heritage of the intelligence service by keeping it closely under their control and, consequently, aligning it with governing party interests. Moreover, a vibrant bottom-up, party-aligned labour union within the service became the main vehicle for the politicisation of the organisational culture. This research uses original oral history interviews with former service personnel, newspaper publications and parliamentary debates on the service between 1974 and 2008. This is the first research on the Greek intelligence service based on such large-scale, longitudinal and diverse empirical data collection. The results of this research are relevant beyond the specific case of Greece, as they point to the wider mechanisms of politicisation of intelligence services, especially in former authoritarian regimes in Southern and Eastern Europe.