The grandson of a slave, he grew up in French Guiana and went to Toulouse to complete his studies. A brilliant student, he became a lawyer in 1918 and worked with César Campinchi, a lawyer who later became an influential politician. He joined the Radical Party and was elected Deputy from French Guiana in 1932. He was Undersecretary of State for the Colonies in the Chautemps government of 1937-1938, becoming the first black man to hold a senior position in the French government.
During the first part of World War II, he served in the French Navy, on the battleship Provence. He was not demobilized till 17 July 1940, well after the French defeat by Germany, and therefore did not vote on the grant of dictatorial powers to Marshal Pétain. He protested against the armistice signed by Pétain, and complained about the treatment of French colonial subjects by Petain’s Vichygovernment. In late 1940, he joined Combat, one of the major groups in the resistance. As a lawyer in Marseille, in unoccupied France, he defended persons arrested or persecuted by the Vichy government for their opinions or racial origin. For this he was repeatedly threatened or arrested by the Vichy police.
When Germany occupied the rest of France in 1942, he went underground, and joined the Maquis of Auvergne, as “Commandant St-Just”. He and his wife Cheylade established a military hospital in June 1944. That fall he was demobilized, and was appointed by the Radical Party to sit in the “Provisional Consultative Assembly” of the restored French government.
In 1945, he was appointed chairman of a commission to determine the future status of the French colonies. In October 1945, he was elected Delegate from French Guiana to the First Constituent Assembly of the Fourth Republic, and to the Second Constituent Assembly in April 1946. Also in 1946, he was a French delegate to the first session of the United Nations. He was defeated for election to the Third Constituent Assembly in November 1946, in part because some Guianans objected to his efforts to close the prison colony of Devil’s Island.
Instead he was named to the Council of the Republic of France (the Senate), which was being reconstituted by appointments. He was immediately elected President of this Council, and became one of the most active members of the Senate. In March 1947, he was chosen President of the Council, by a vote of 141 to 131 over the Communist candidate.
In 1948, he changed his residence from Guiana to Lot, and was elected Senator there. He served as Senator from Lot and President of the Council until the end of the Fourth Republic in 1958.
In 1958, Monnerville supported Charles de Gaulle in returning to power, but he objected to De Gaulle’s dissolution of the Fourth Republic. However, when the Fifth Republic was established, he resumed his place in the Senate (now called by that name), and was elected President of the Senate in 1959, serving until 1968.
In 1962, he famously opposed the referendum altering the constitution for changing the method of election of the president to a direct election, instead of an electoral college, on grounds that the method for constitutional amendments was not respected, a reform strongly desired by Charles de Gaulle. The Constitutional Council however ruled itself “incompetent” to strike down a reform voted by the French people. He went as far as to use the strong word of forfaiture (“abuse of authority”) against the behaviour of Prime Minister Georges Pompidou, who had accepted to sign the referendum project 
From 1977 to 1983, he was a member of the Constitutional Council of France.