He was born in Lyon, France, on 11 November 1905, into a Catholic family with a military tradition. He studied the Germanic languages at the University of Strasbourg. Afterwards, he became a soldier like his father and studied in Saint Cyr and the École Supérieure de Guerre and reached the rank of captain in 1934. At the outbreak of World War II, he rejoined the French army. German forces captured him in Vosges. He arrived in Marseille after escaping from a POW camp in Alsace on 27 June 1940.
At first Frenay supported the Vichy Regime but was soon disillusioned by the Nazi tendency of the Pétain regime, and he subsequently formed the French Resistance group Mouvement de Libération Nationale in 1940. He became an editor of underground newspapers such as Vérités (Truths) and had a hand in the formation of the Combat group in November 1941. In 1943, his group participated in the forming of the Conseil National de la Résistance of Jean Moulin, but Frenay refused a seat since he disagreed over the admission of political parties to the Conseil.
After the war, Frenay served in de Gaulle's first provisional government. He retired from the political life and became a businessman. He published his autobiography, The Night Will End: Memoirs of a Revolutionary in 1976 and criticised Moulin and de Gaulle as reckless.