[2][3] When World War II began in September 1939, Le Héros was assigned to the 1st Submarine Division based at Toulon, France.

[2][4][5] In December 1939, Le Héros was sent in search of the German tanker Altmark in the mid-Atlantic Ocean with her sister ships Achéron, Agosta, Bévéziers, Fresnel, and Redoutable.

[2][4][6] From 8 to 29 March 1940, she carried out a patrol in the South Atlantic Ocean between Fernando de Noronha and the coast of Brazil, without success.

[4] Many members of their crews initially refused to fight the British, but the commanding officer of Le Héros convinced them to participate in the sortie.

[4] In either event, both submarines spent the day submerged off Dakar[4] and attempted to attack the British ships there but could not get close enough, and the British departed the area and headed for Freetown after Fleet Air Arm Fairey Swordfish torpedo bombers from the aircraft carrier HMS Hermes attacked and seriously damaged the French battleship Richelieu at Dakar that day.

[8] In October 1941, the British boarded and captured a convoy of four Vichy French cargo ships en route to Dakar.

[8][note 2] On either 15 or 16 November 1941, according to different sources,[8] one of the submarines attacked the 4,000-gross register ton cargo ship Capo Olmo, some sources claiming that Le Glorieux fired two torpedoes at her south of the Cape of Good Hope off Port Elizabeth, South Africa, at 35°40′S 019°20′E / 35.667°S 19.333°E / -35.667; 19.333 and that both missed,[8][12] others that Le Héros made the attack during a storm on 16 November 1940 and claimed to have sunk Capo Olmo.

[2] After arriving at Djibouti, Le Héros conducted a patrol in the Gulf of Tadjoura off French Somaliland from 27 February to 2 March 1942.

[2][4][13] In the Indian Ocean 500 nautical miles (926 km; 575 mi) north of Madagascar at the time, Le Héros accompanied the cargo ship she was escorting as it altered course to steer for the Comoro Islands.

[2][4] At 23:15 on 6 May 1942, she was 5 nautical miles (9.3 km; 5.8 mi) off Madagascar, near Cap d'Ambre (the northern tip of the island) and east-northeast of the Alizès Pass.

[2][4] At 03:10 on 7 May 1942, Le Héros passed around the northern tip of Madagascar,[2] heading for Courrier Bay on the northwestern coast to attack the British invasion fleet there.

[2] At dawn, a flight of eight Fairey Swordfish torpedo bombers from the aircraft carrier HMS Illustrious surprised her on the surface either north or west of Courrier Bay, according to different sources, and attacked her with depth charges.

[2][3] She crash-dived, but a violent explosion shook her, knocking out her electrical power, rupturing her compressed air and diesel fuel systems, damaging her forward diving planes, and creating a leak which allowed seawater to enter her forward battery room, causing the release of poisonous chlorine gas.

[2][4][3] Le Héros′s entire crew of 72 men was left drifting in rough seas, and those without lifebelts quickly became exhausted and sank to their deaths.