He was born on 13 January 1982 in Zagreb, the capital of Croatia. He graduated from the Faculty of Political Science at the University of Zagreb in 2006. In 2013, he completed his master’s degree in the fields of environment, society, and development at the University of Cambridge in the United Kingdom.

Tomašević became involved in environmental movements at a young age, joining the environmental organization Green Action at the age of 16 and serving as its president between 2007 and 2012. He also held positions such as President of the Croatian Youth Network and member of the Croatian Government’s Youth Council. In 2006, he took part in the establishment of the “Right to the City” initiative and served on the Croatian Government Council for Sustainable Development and Environmental Protection from 2012 to 2015.

In 2017, he became one of the founders of the political movement Zagreb Je Naš! (“Zagreb Is Ours!”) and participated in the 2017 Zagreb City Assembly elections on the movement’s list. In the second round of the local elections held on 30 May 2021, he was elected Mayor of Zagreb with 63.87% of the vote as the candidate of the Možemo! (“We Can!”) and Zagreb Je Naš! coalition. This result represents the highest vote share ever received by a directly elected mayor in Zagreb. He officially assumed office on 4 June 2021.

In the second round of the local elections held on 1 June 2025, he was re-elected Mayor of Zagreb with 57.56% of the vote.

During his term as mayor, he has prioritized projects aimed at enhancing the city’s sustainability. These initiatives include the installation of solar panels on building rooftops, the implementation of energy-efficiency measures, and the adoption of circular economy principles. He has also focused on waste and water management, transportation, and the fairer distribution of urban services.

Tomašević is known for his commitment to the values of participatory democracy, environmental protection, social equality, and transparent governance. Throughout his political career, he has sought to encourage public participation and make city administration more transparent.