Mimica was elected to the Croatian Parliament in the 2003 elections as a representative of the Social Democratic Party, and again in 2007. From January 2008 he served as deputy speaker of the Croatian parliament and also as the chairman of the parliament's Committee for European Integration. In addition to his parliamentary work, Mimica served as consultant to the Government of Kosovo on establishing European integration structures from 2005 until 2006.
Deputy Prime Minister, 2011–2013
Ahead of the 2011 elections, SDP chairman Zoran Milanović assigned key preparatory work to Mimica, including the task of drafting public administration bills that later formed the basis of Milanovic's reform agenda. On December 23, 2011 he was selected as one of four Deputy Prime Ministers in the government of Milanović, responsible for internal, foreign and European policy.
On July 1, 2013 Mimica became European Commissioner for Consumer Protection in the European Commission. In this capacity, he was in charge of consumer markets, health technology and cosmetics, product and service safety, financial services and redress, and consumer strategy and enforcement. For the remainder of his time in office, he shepherded through the Council of Ministers and European Parliament draft legislation that had already been proposed. He also oversaw the implementation of existing legislation. László Andor was Acting Commissioner in his stead, from 19 April 2014 – 25 May 2014 while he was on electoral campaign leave for the 2014 elections to the European Parliament. He ultimately decided to not take up his seat. From November 1, 2014 Mimica was European Commissioner for International Cooperation and Development in the European Commission led by Jean-Claude Juncker. In this capacity, he is part of the Commission's foreign policy around Federica Mogherini. Early in Mimica's tenure, the European Union approved 1.15 billion euros in aid for West Africa as part of the five-year European Development Fund program in 2015, nearly doubling its previous commitment to a region that is a major source of migrants seeking to enter Europe. In late 2015, he negotiated a 200 euro million aid agreement with Eritrea as part of the Commission's efforts to reduce the flow from what was, at the time, the source of the third largest number of migrants. Since 2016, he has been working to implement the European Union's 44 billion euro Emergency Trust Fund for Africa which is aimed to entice private investors to some of the world's poorest nations and slow mass migration to Europe. Also during Mimica's time in office, the European Union suspended direct financial support for the government of Burundi’s PresidentPierre Nkurunziza after concluding that it had not done enough to find a political solution to the 2015 post-election unrest in the country. The European Union funds about half the annual budget of Burundi and has imposed sanctions on officials close to the president. In September 2016, Mimica was appointed by United Nations Secretary-GeneralBan Ki-moon to serve as member of the Lead Group of the Scaling Up Nutrition Movement. Towards the end of his term, in 2019, Mimica pledged a total of 500 million euros in contributions of the EU to Global Fund to Fight AIDS, Tuberculosis and Malaria for the 2020-2022 period.