In 1986, he joined Czechoslovakia's Foreign Ministry as an Assistant Legal Adviser, and in 1990 was promoted to Head of the Ministry's Public International Law Division. The following year, he was transferred to the country's United Nations Mission, where he served as Legal Adviser. Following the division of Czechoslovakia, he served as Slovakia's Deputy Permanent Representative from 1993 to 1994. From 1994 to 1997, he served as Slovakia's Ambassador to the United Nations. Returning to the Foreign Ministry, he served as Director of the International Law Department from 1997 to 1998, when he transferred to the post of Director-General for International Legal and Consular Affairs. After a year in that post, he was reappointed as Slovakia's UN Ambassador, serving until his appointment to the Court. He also held a seat on the International Law Commission between 1999 and 2002. He is member of the Slovak Society of International Law and has served as the organization's Honorary President since 2003. He is also a member of the American Society of International Law and the European Society of International Law.