It is noteworthy that the National Assembly actively participated in the negotiations on the text of the Lisbon Treaty. The Slovenian stance on the draft was coordinated and accepted not only within the government but also in the parliament. After joining the EU, all positions represented by the government of the Republic of Slovenia in the Council of the EU and the European Council are generally considered and decided upon by the National Assembly. Typically, discussions and decisions take place at the level of the Committee on European Union Affairs, while positions regarding the EU's foreign and security policy are discussed at the Committee on Foreign Policy. At the plenary level, the National Assembly adopts a declaration outlining Slovenia's priorities in EU institutions, defining the country's focus areas during its EU membership. The latest such declaration was adopted in March 2021 for the period 2021–2024.

Similarly, at the plenary level, the National Assembly also articulates Slovenia's positions on amendments to the EU’s fundamental treaties and, upon their signing, ratification laws. Since Slovenia's EU membership commencement, the National Assembly has adopted three such positions: two related to accession treaties (April 22, 2005, regarding the accession of the Republic of Bulgaria and Romania to the EU; October 19, 2011, regarding the accession of the Republic of Croatia to the EU – supporting the signing in both cases) and one (October 12, 2007) related to the draft Treaty amending the Treaty on European Union and the Treaty establishing the European Community (Lisbon Treaty).

In its position on the draft of the latter treaty, it emphasized Slovenia's commitment, as a state that ratified the Treaty establishing a Constitution for Europe, to preserving institutional and functional gains from that treaty to the greatest extent. It particularly highlighted the legally binding nature of the Charter of Fundamental Rights, the unified legal personality of the EU, the elimination of the tripartite structure, the preservation of institutional balance in a broader sense, and references to cultural and linguistic diversity.

Because the Lisbon Treaty stipulates that national parliaments actively contribute to the good functioning of the EU, on October 20, 2010, the National Assembly also adopted amendments to its rules of procedure. Its most significant competence is associated with overseeing the observance of the subsidiarity principle in proposals for EU legislative acts.