Lamas, born in Buenos Aires into the Argentinian aristocracy, was an outstanding student who received his doctorate in laws from the University of Buenos Aires, and then began a career as a teacher of law and sociology at the University of La Plata, eventually rising to the position of professor at Buenos Aires.
As an academic, some of his major published works concentrated on labor law. He was involved in the early stages of the International Labor Organization (ILO), and much of his work ended up being the basis of Argentina's own labor laws.
His political career began in 1906 through a succession of steadily more important roles, including two terms in the Argentinian parliament beginning in 1908, where his interests were mainly in foreign affairs. In 1915, he became Minister for Justice and Education.
The achievement for which he received the Nobel Prize was as Argentina's foreign minister, which he was from 1932 to 1938, mediating a treaty to end the Chaco War between Paraguay and Bolivia and instigating several multilateral nonaggression treaties between a number of South American countries.
Lamas married the daughter of a former Argentinian president.
From 1941 to 1943 he was president of the University of Buenos Aires, and then professor until 1946.