In the account of Dares the Phrygian, Diomedes was illustrated as "stocky, brave, dignified, and austere. He was loud at the war-cry, hot-tempered, impatient, and daring."

Diomedes was, on his father's side, an Aetolian, and on his mother's an Argive. His father, Tydeus, was himself of royal blood, being the son of Oeneus, the king of Calydon. Tydeus had been exiled from his homeland for killing relatives, and found refuge at Argos, where King Adrastus offered him hospitality and gave him his daughter Deipyle to be his wife. The two had two children together — a daughter, Comaetho, and a son, Diomedes.

At the same time the banished Theban prince Polynices arrived in Argos, and Adrastus gave him his daughter Argia. Polynices begged Adrastus' aid to recover his homeland, and Adrastus gathered an expeditionary force of seven leaders, each to assault one of the seven gates of Thebes; Tydeus, Polynices and Adrastus were among them. These champions were known as the Seven against Thebes. The expedition failed: all seven Argive champions were killed except Adrastus, who escaped on his swift horse Arion. Diomedes' father Tydeus was among the slain.

Tydeus was Athena's favorite warrior, and as he lay dying she wished to give him a medicine that would make him immortal; but she withdrew the gift in disgust when Tydeus gobbled down the brains of the enemy who had wounded him. Diomedes was about four years old when his father was killed.

At the funeral of their fathers, the sons of the seven fallen champions — Aegialeus, Alcmaeon, Amphilocus, Diomedes, Euryalus, Promachus, Sthenelus, and Thersander — vowed to vanquish Thebes to avenge their fathers. These sons were known as the Epigoni ("offspring"). Ten years later they launched a second expedition against Thebes under Alcmaeon's command. The main battle took place at Glisas, where prince Aegialeus was slain by King Laodamas, who was in turn killed by Alcmaeon. The Thebans, on the advice of the seer Tiresias, fled the city; the Epigoni entered, plundered its treasures and tore down its walls, then installed Thersander, son of Polynices, as its new ruler.

As the Argive forces travelled home, the elderly Adrastus died of grief upon learning his son Aegialeus had perished, leaving Diomedes as the last of Adrastus' male descendants. On returning to Argos, Diomedes ascended the throne, and to secure it he married Aegialeus' daughter, Princess Aegialia. He ruled Argos well, bringing it wealth and stability, and was respected by other rulers.

Diomedes still kept watch on Calydonian politics, his father's homeland. When the sons of Agrius (led by Thersites) imprisoned Oeneus, Diomedes' grandfather, and set their own father on the throne, Diomedes resolved to restore Oeneus. He seized the kingdom and slew the traitors except Thersites, Onchestus (who escaped) and Agrius (who killed himself), restoring his grandfather. Oeneus later passed the kingdom to his son-in-law Andraemon and set out for Argos, but was murdered on the way in Arcadia by Thersites and Onchestus. Unable to find the killers, Diomedes founded a city, Oenoe, at his grandfather's burial place to honour him.

According to Hyginus and Pseudo-Apollodorus, Diomedes was one of the suitors of Helen and so was bound by the oath of Tyndareus to defend her chosen husband; thus, when Paris stole Menelaus' wife, Diomedes was summoned by Agamemnon for the expedition to Troy. Hesiod, however, does not include Diomedes among the suitors, and his suitor status may be a later inference from his appearance in the Catalogue of Ships — for if Helen ruled Sparta for ten years before her abduction, Diomedes would have been a child at the time of her marriage.

Diomedes is known primarily for the Trojan War. According to Homer he entered the war with a fleet of 80 ships, third only to Agamemnon (100) and Nestor (90), leading the troops from Argos, Tiryns, Troezen and other cities; Sthenelus drove his chariot and was probably his closest friend. Though the youngest of the Achaean kings, he was among the most experienced, having fought in the war of the Epigoni, and is reckoned second only to Achilles as the mightiest and most skilled Achaean warrior. He received the most direct divine help: he was Athena's favorite (she once drove his chariot herself), and apart from Heracles he was the only hero to attack the Olympian gods, even wounding Ares. His golden armour, blessed by Athena, bore the crest of a boar; his cuirass was made by Hephaestus.

At Aulis, Diomedes met his brother-in-arms Odysseus, with whom he shared many adventures; each embodied an aspect of their patron Athena — Odysseus her cunning, Diomedes her courage and skill. According to Hyginus, Diomedes accompanied Odysseus to fetch Iphigenia for her sacrifice (other sources name different envoys). In the matter of Palamedes, some traditions say Diomedes joined Odysseus in drowning or stoning him; others say he merely knew of the plot and did not intervene, because Odysseus was essential to Troy's fall.

Diomedes is one of the central figures of the Iliad, the key fighter of its first third, often presented as the perfect embodiment of heroic virtue: striving for glory while avoiding hubris, retreating before Ares and giving way to Apollo so as to remain within mortal limits. He is repeatedly shown to possess wisdom respected by older comrades; in Book IV he answers Agamemnon's taunt with restraint, and in Book IX he rebukes Agamemnon's proposal to abandon the war, predicting (correctly) that Achilles will fight again whatever he chooses, since human will cannot defy fate.

Diomedes' aristeia — the longest in the epic — begins in Book V, where Athena puts valour in his heart and makes fire flare from his shield and helmet, granting him special vision to distinguish gods from men with orders to wound Aphrodite. He slaughters many Trojans, crushes Aeneas' hip with a great stone, and wounds Aphrodite as she shields her son; Apollo rescues Aeneas, warning Diomedes three times not to match himself against immortals. When Ares enters the battle, Athena mounts Diomedes' chariot and guides his spear into the war-god's belly, making him the only human to wound two Olympians in a single day. In Book VI he meets the Trojan Glaucus between the lines; discovering that his grandfather Oeneus had once hosted Glaucus' ancestor Bellerophon, the two refuse to fight and exchange armour — Diomedes giving bronze worth nine oxen for Glaucus' gold worth a hundred, the origin of the phrase "Diomedian swap."

Through the later books Diomedes remains prominent: he speaks first in council, urges continued war, rescues the stranded Nestor in Book VIII, and twice nearly kills Hector in Book XI before being wounded in the foot by Paris' arrow and withdrawing. In Book X (the Doloneia), he volunteers for a night raid and chooses Odysseus as his companion; together they capture and behead the Trojan spy Dolon, then slaughter the sleeping Thracian king Rhesus and twelve of his men and carry off his famous white horses — fulfilling a prophecy that Troy could not fall while those horses fed on its plains. In the funeral games of Patroclus (Book XXIII) Diomedes wins the chariot race with the horses he took from Aeneas and draws first blood against Ajax the Great in the armed sparring contest, asserting his status as the foremost Achaean after Achilles. Athena is consistently closest to him, aiding him without being asked and appearing to him undisguised in her true form.

In the Posthomerica, Diomedes slays two Amazon warriors of Penthesileia and afterward, alone among the leaders, mourns his abusive cousin Thersites when Achilles kills him, nearly coming to blows with Achilles before being reconciled. With Odysseus he fetches Neoptolemus from Scyros and retrieves Philoctetes and the bow of Heracles from Lemnos — needed for Troy's fall. After Helenus reveals that Troy cannot be taken while the Palladium (a statue of Athena) remains inside, Diomedes and Odysseus steal it; Diomedes is generally regarded as the one who physically removed it and carried it to the ships. On the way back, when Odysseus plotted to kill him and claim the prize, Diomedes saw the gleam of the sword by moonlight, disarmed and bound him, and drove him before him — but spared him as essential to Troy's destruction, giving rise to the proverb "Diomedes' necessity." Diomedes was also one of the warriors inside the Wooden Horse, slaying many Trojans during the city's fall.

During the sack of Troy, the Achaean leaders failed to punish Ajax the Lesser for his sacrilege against Cassandra at Athena's altar, earning the goddess's wrath. Diomedes and Nestor decided to leave Troy immediately and reached home safely, Diomedes arriving in Argos only four days after his departure; his safe nostos implies he kept the favour of the gods, unlike the many Achaeans wrecked by the storm Athena and Poseidon raised.

At home, however, the Palamedes affair pursued him. Palamedes' brother Oeax reported to Diomedes' wife Aegialia that her husband was bringing back a woman he preferred to her; other versions say Aegialia had taken a lover at the urging of Palamedes' father Nauplius, or that Aphrodite — who never forgave the wound Diomedes had dealt her at Troy — gave Aegialia many lovers. In any case Aegialia, helped by the Argives, kept Diomedes from entering the city, and he fled by night with his companions.

Diomedes then migrated through Aetolia to Daunia (Apulia) in Italy, to the court of King Daunus, whom he helped defeat the Messapians in exchange for land and marriage to the king's daughter Euippe, by whom he had two sons, Diomedes and Amphinomus. He built the city of Argyrippa and once hosted Aeneas there; when Aeneas asked for the Palladium, Diomedes returned it, because an oracle had told him to give it to a Trojan to end his misfortunes. In another tale a storm drove him to Libya, where King Lycus imprisoned him to sacrifice him to Ares until the king's daughter Callirrhoe freed him — and, abandoned by him, killed herself out of grief.

The Greeks and Romans credited Diomedes with founding many settlements in Magna Graecia in southern Italy, including Argyrippa (Arpi), Beneventum, Brundusium, Canusium, Venafrum, Salapia, Spina, Sipus, Ancona and Venusia, the last made as a peace-offering to Aphrodite. Virgil's Aeneid describes the prosperity of his kingdom and his refusal to aid Turnus against Aeneas, declaring he had fought enough Trojans and wished only to live in peace. Diomedes spread the worship of gods and heroes widely: he built temples of Athena in and near Argos, dedicated his armour at Luceria, and founded a temple of Apollo Epibaterius and the Pythian games at Troezen.

Hero cults of Diomedes, widespread from the 8th century BCE, were concentrated in Cyprus, Metapontum and the cities of the Adriatic coast, the chief votive evidence coming from the island of Palagruža. He was honoured as a hero at Thurii and Metapontum, and at Argos his shield was carried through the streets with the Palladium during the festival of Athena while his statue was washed in the river Inachus. Two Adriatic islands, the Islands of Diomedes, were named after him, and Strabo reports that the Heneti sacrificed a white horse to him in groves where wild animals grew tame.

Strabo lists four traditions about the end of Diomedes' life in Italy: that he was making a canal at Urium when summoned home to Argos and died there, leaving his work unfinished; that he stayed at Urium until the end of his life; that he vanished on the uninhabited Adriatic island of Diomedea, implying a kind of deification; and that, according to the Heneti, he remained in their country and underwent a mysterious apotheosis.

Several legends connect his death with birds: on his death the albatrosses are said to have sung, or his companions were turned into birds. The family name for albatrosses, Diomedeidae, and the genus name for great albatrosses, Diomedea, derive from him. In another story a shipwreck stranded his men on an Adriatic island, Diomedes was killed by Aeneas, and his men became birds that guarded his shrine. On San Nicola Island in the Tremiti Archipelago lies a Hellenic tomb called Diomedes's Tomb, where, by legend, Venus transformed his grieving men into birds to keep watch over their king.

According to post-Homeric stories, Diomedes was given the immortality by Athena that she had withheld from his father; Pindar in Nemean X says "the golden-haired, gray-eyed goddess made Diomedes an immortal god." A scholiast adds that to attain immortality he married Hermione, daughter of Menelaus and Helen, and lives with the Dioscuri as a god while enjoying honours at Metapontum and Thurii. He was worshipped under various names in Italy, with statues at Argyripa, Metapontum and Thurii and a temple at the Adriatic.

Less common versions place him in the afterlife: a drinking song to the tyrannicide Harmodius names Diomedes among the inhabitants of the Islands of the Blessed, alongside Achilles. In Dante's Inferno, by contrast, Diomedes is seen in the Eighth Circle of Hell among the counsellors of fraud, punished in sheets of flame with Odysseus for advising the theft of the Palladium and the stratagem of the Trojan Horse.

Diomedes plays an important role in the medieval legend of Troilus and Cressida, in which he becomes Cressida's new lover after she is sent to the Greek camp to join her traitorous father. In Shakespeare's play Troilus and Cressida, Diomedes is often seen fighting Troilus over her.