Professor Lapaš was born in Zagreb in 1971. He graduated from the University of Zagreb, Faculty of Law in 1995. He has worked at the International Law Department of the Zagreb Faculty of Law since 1998. At the same Faculty he completed the LL.M. Studies in Public and Private International Law in 1999, and defended his Ph.D. thesis in 2003.
In 2008 he was elected as associate professor of Public International Law, and in 2012 as full professor of Public International Law at the Zagreb Faculty of Law, Department of International Law.
He attended legal courses and seminars in Public International Law at the Hague Academy of International Law in 1997; at the Asser Institute for Private and Public International Law in The Hague in 1998; in Aix-en-Provence in 1999; at the UN International Law Commission Seminar in Geneva in 2000; at the Institute of International Public Law and International Relations in Thessaloniki in 2000; and at the Max Planck Institute for Comparative Public Law and International Law in Heidelberg in 2002.
He teaches Public International Law, Diplomatic and Consular Law, Development of International Law and Law of International Organizations at the Faculty of Law, University of Zagreb. He also teaches at the Postgraduate Doctoral Studies at the Zagreb Faculty of Law, at the Postgraduate Specialist Studies in Foreign Policy and Diplomacy and at the Postgraduate Doctoral Studies in International Relations at the Zagreb Faculty of Political Sciences, as well as at the Postgraduate Specialist Studies in Diplomacy and at the Diplomatic Academy of the Ministry of Foreign and European Affairs of the Republic of Croatia.
He participates in research projects of the Ministry of Science of the Republic of Croatia (researcher no. 219494).
He is a Croatian alternate member of the Court of Conciliation and Arbitration within the OSCE
In 2014 he was Co-Agent of the Republic of Croatia before the International Court of Justice in the case concerning Application of the Convention on the Prevention and Punishment of the Crime of Genocide (Croatia v. Serbia).