After serving four years in the Russian foreign ministry, Martens taught public law in St. Petersburg from 1872 to 1905. He helped to settle the controversy between Great Britain and France over Newfoundland (1891) and a Mexican–U.S. dispute that was the first case determined by the Permanent Court of Arbitration, The Hague (1902). During the Russo-Japanese War (1904–05), he took part in the negotiations that led to the peace treaty of Portsmouth, N.H. (Sept. 5, 1905). Representing Russia at the second international conference at The Hague (1907), he served as president of the maritime law committee.

Martens wrote books on the right of private property in war (1869); the expansion of Russia and Great Britain in Central Asia (1879); international law (1882); and the Berlin conference of 1884–85, concerning European spheres of influence in Africa, the Middle East, China, and the Pacific (1887). His most ambitious work of editing, Recueil des traités et conventions conclus par la Russie . . . (15 vol., 1874–1909), contains not only the texts of treaties between Russia and other countries but also histories (based on unpublished Russian documents) of the diplomatic conditions necessitating the treaties. It was printed in Russian and French in parallel columns.