[20] He has represented different states on at least twelve occasions before the ICJ, including Argentina in the Uruguay River pulp mill dispute and the Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965; Colombia in the Case concerning The Territorial and Maritime Dispute (Nicaragua v. Colombia); Costa Rica in the Dispute regarding Navigational and Related Rights (Costa Rica v. Nicaragua); and Serbia in the Advisory opinion on Kosovo's declaration of independence.

[12][13] Kohen is often cited in English, French, and Spanish-language news media for his expertise on matters of statehood, secession, and peaceful settlement of international disputes.

[27][28] His book "Possession contestée et souveraineté territoriale" (Presses universitaires de France, 1997) was awarded the Paul Guggenheim Prize in 1997.

[29][30] Marcelo Kohen was elected Secretary General of the Institute of International Law in 2015, at the 77th session held in Tallinn (Estonia).

In this context, he argued that "the law is the weapon of the weak in their relationship with the powerful" and stressed that "force does not give rights.