This article assesses the development of Mercosur’s institutions across its 30 years of history. It aims to stress how the insertion of supranational instances in the bloc was historically disregarded by Member States, in the context of both right and left-wings governments. However, the creation of a Technical Secretariat, a Permanent Review Tribunal, and a regional parliament (Parlasur) institutionalized non-executive forums, which have become autonomous regional arenas, despite their marginalized position within the bloc’s decision-making system. Although these bodies have never seriously challenged Mercosur’s intergovernmental, and even interpresidential, institutional design, they have enabled the bloc to expand its agenda beyond the governments’ priorities. Thus, this article aimed to unveil the causes of Mercosur’s resistance to supranational institutional change. The qualitative methodological approach is based on specialized literature, but also draws on primary sources and the normative analysis of official documents and reports which have gone through a deductive assessment. First, the article will introduce the main institutional changes seen in Mercosur during its 30 years of existence. Secondly, we argue that these transformations have maintained Mercosur’s intergovernmentalism as its main institutional feature, although additional non-executive bodies were set up in the 2000s. Afterwards, it reflects upon the current circumstances of the bloc, addressing whether future institutional reforms would alter Mercosur’s structural configurations.