Hieronymus king of Syracuse, succeeded his grandfather, Hieron II., in 216 BC. He was at this time only fifteen years old, and he ascended the throne at a crisis full of peril, for the battle of Cannae had given a shock to the Roman power, the influence of which had been felt in Sicily; and though it had not shaken the fidelity of the aged Hieron, yet a large party at Syracuse was already disposed to abandon the alliance of Rome for that of Carthage. The young prince had already given indications of weakness, if not depravity of disposition, which had alarmed his grandfather, and caused him to confide the guardianship of Hieronymus to a council of fifteen persons, among whom were his two sons-in-law, Andranodorus and Zoïppus. But the objects of this arrangement were quickly frustrated by the ambition of Andranodorus, who, in order to get rid of the interference of his colleagues, persuaded the young king to assume the reins of government, and himself set the example of resigning his office, which was followed by the other guardians. Hieronymus now became a mere tool in the hands of his two uncles, both of whom were favourable to the Carthaginian alliance : and Thrason, the only one of his counsellors who retained any influence over his mind, and who was a staunch friend of the Romans, was soon got rid of by a charge of conspiracy. The young king now sent ambassadors to Hannibal, and the envoys of that general, Hippocrates and Epicydes, were welcomed at Syracuse with the highest honours. On the other hand, the deputies sent by Appius Claudius, the Roman praetor in Sicily, were treated with the utmost contempt ; and it was evident that Hieronymus was preparing for immediate hostilities. He sent ambassadors to Carthage, to conclude a treaty with that power, by the terms of which the river Himera was to be the boundary between the Carthaginians and Syracusans in Sicily : but he quickly raised his demands, and, by a second embassy, laid claim to the whole island for himself. The Carthaginians readily promised every thing, in order to secure his alliance for the moment : and he assembled an army of fifteen thousand men, with which he was preparing to take the field, having previously dispatched Hippocrates and Epicydes to sound the disposition of the cities subject to Rome, when his schemes were suddenly brought to a close. A band of conspirators, at the head of whom was Deinomenes, fell upon him in the streets of Leontini, and dispatched him with numerous wounds. before his guards could come to his succour, 215 BC. (Liv. xxiv. 4-7; Polyb. vii. 2-6.)

The short reign of Hieronymus, which had lasted only 13 months, had presented the most striking contrast to that of his grandfather. Brought up in the midst of all the enervating and corrupting influences of a court, his naturally bad disposition, at once weak and violent, felt them all in their full force; and he exhibited to the Greeks the first instance of a childish tyrant. From the moment of his accession he gave himself up to the influence of flatterers, who urged him to the vilest excesses : lie assumed at once all the external pomp of royalty which Hieron had so studiously avoided; and while he plunged in the most shameless manner into every species of luxury and debauchery, he displayed the most unrelenting cruelty towards all those who became objects of his suspicion. Polybius indeed appears inclined to doubt the statements on this subject; and it is not improbable that they may have been exaggerated by the writers to whom lie refers : but there is certainly nothing in the nature of the case to justify his scepticism; and the example, in later days, of Elagabalus, to whose character that of Hieronymus appears to have borne much resemblance, is sufficient to show how little any excesses that are reported of the latter can be called incredible. Among other instances of his wanton contempt of public decency, he is said to have married a common prostitute, on whom he bestowed the title and honours of a queen. (Polyb. vii. 7; Liv. xxiv. 5 ; Diod. Exc. Vales. xxvi. p. 568, 569; Athen. vi. p. 251, xiii. p. 577; Val. Max. iii. 3. Ext. § 5.)

The coins of Hieronymus are more abundant than might have been expected from the shortness of his reign : they all bear his portrait on the obverse, and a thunderbolt on the reverse.