Sthenelus' father Capaneus was one of the Seven Against Thebes. He was an outstanding warrior, but he was also notorious for his arrogance. He stood just at the wall of Thebes during the war of the Seven against Thebes and shouted that Zeus himself could not stop him from invading it. While he was mounting the ladder, Zeus struck and killed Capaneus with a thunderbolt. At his funeral, Sthenelus watched as his mother Evadne threw herself on her husband's funeral pyre and died.[9]
The sons of the Seven Against Thebes, including Sthenelus, swore to avenge their fathers, after which they were called the Epigoni. Ten years later, they defeated the Thebans and took the city. Sthenelus ruled Iphis' half of Argos, along with Diomedes, after both Adrastus and Aegialeus had died.[10]
Sthenelus fought alongside Diomedes and the other Argives in the Trojan War and brought 25 ships to Troy.[7] In the Iliad, Agamemnon insults Sthenelus and Diomedes by comparing them to their fathers. While Diomedes keeps his composure, Sthenelus cannot contain his anger. He boasts that they are better than their fathers, because they were able to capture Thebes when their fathers had not been able to do so. This may indicate that he, like his father, is arrogant. However, when Diomedes advises him not to argue with Agamemnon further, he heeds Diomedes' counsel, showing that he is capable of thinking rationally, even in the heat of anger.[11]
The Iliad portrays Sthenelus and Diomedes as close companions both on and off the battlefield. Sthenelus drives Diomedes' chariot and advises him in battle. Their close relationship is emphasized by Diomedes when he proclaims that, even if all the other Achaeans lose faith and return home, he and Sthenelus will stand together and fight until Troy falls.[12]
When Diomedes returned to Argos after the fall of Troy, he learned that his wife Aegiale was having an affair with Cometes, the son of Sthenelus. They plotted to kill Diomedes, but he was able to escape and travel to Italy. It is unknown if Sthenelus joined him or stayed in Argos. He was succeeded by his son Cylarabes.[14] During Cylarabes' reign, Argos was finally reunited after having been divided into three parts since the reign of Anaxagoras.[15]