In 1884, when he was 19 years old, Bustamante won a public competitive examination for the professorship of international law at the University of Havana. From 1902 (when the Republic of Cuba was constituted) until 1918 he was a Cuban senator. He represented Cuba at the second international conference at The Hague (1907) and at the World War I peace conference in Paris (1919). In 1908 he was made a member of the Permanent Court of Arbitration, The Hague, and in 1921 he became a judge of the Permanent Court of International Justice, newly established by the League of Nations.

Bustamante wrote numerous books, including Tratado de derecho internacional privado (1896; “Treatise on International Private Law”); El Tribunal Permanente de Justicia Internacional (1925; The World Court, 1925); and Derecho internacional público, 5 vol. (1933–38; “International Public Law”).