Lammasch promoted international law and arbitration. He became associated with the Institute of International Law in 1887, and a member in 1891. He played an important role at international conferences and meetings and he was member of the Permanent Court of Arbitration at The Hague. Lammasch presided over the Court in the conflicts between the USA and Great Britain over the fishery in New Foundland (1909), and between the USA and Venezuela over the rights of the Orinoco Steamship Company (1910). He became a lifelong member of the Austrian First Chamber in 1899 and Hon. member of the Vienna Academic Peace Association. Member of the Central Organization for a Durable Peace (founded in 1915). After 1916 Lammasch continued to produce works on peace, international law and arbitration, despite deteriorating health.