Karlo Ressler was born on 26 December 1989 in Zagreb. After finishing elementary school and high school, he enrolled in the Faculty of Law in Zagreb. Karlo Ressler graduated with honours (magna cum laude) from the Faculty of Law in Zagreb. He completed his senior year at the University of Sheffield, England, participating in an Erasmus Student Exchange programme.
He is currently completing his doctorate degree on Human Trafficking in South East Europe at the Max Planck Institute for International and Foreign Criminal Law in Freiburg, Germany.
After graduation, Ressler started working as an assistant at the Faculty of Law in Zagreb. From 2013 to 2016, he was an assistant to Andrej Plenković, who was a Member of the European Parliament at that time. When the Croatian Democratic Union (HDZ) won the parliamentary elections in autumn 2016, he was appointed as an advisor for legal and political issues to Prime Minister Plenković.
Since 2009, Karlo Ressler is a Member of the Croatian Democratic Union (HDZ). In 2017, he led the Working Group on the new Statute of the Croatian Democratic Union (HDZ). He is a Member of the HDZ Statute Commission and Vice-President of the Party's Judicial Committee. Ressler coordinated the creation of the HDZ Programme "Croatia for Generations" for the 2019 European Parliament elections.
He is the Vice-President of the Youth of the European People's Party (YEPP) for the second term.
In May 2019, he led the HDZ list for the European elections. The Croatian Democratic Union (HDZ) won 4 mandates and the highest number of votes, with 22.7 percent. Karlo Ressler won the highest number of preferential votes on the HDZ list, with 52.309 votes, equating to 21.71 percent, the fourth best overall score.
Within the European Parliament, Karlo Ressler is a Member of the Committee on Budgets (BUDG) and a Substitute Member of the Committee on Civil Liberties, Justice and Home Affairs (LIBE).
MEP since 2019.