He was the son of Dionysius the Elder. When his father died in 367 BC, Dionysius assumed rule under supervision of his uncle, the philosopher Dion. Dion's disapproval of the young Dionysius' lavishly dissolute lifestyle compelled him to invite his teacher Plato to visit Syracuse. Together they attempted to restructure the government as more moderate, with Dionysius as the archetypal philosopher-king (see the Seventh Letter of Plato).
Angered by the philosophers' attempt, Dionysius conspired with the historian Philistus and banished his uncle, taking complete rule in 366 BC. Without Dion, Dionysius' rule became increasingly unpopular, as he was mostly incompetent in governing men and commanding soldiers. When Plato appealed for Dion's return, the irritated Dionysius interfered with Dion's property and finances and gave his wife to another man. Before this, Dion had lived comfortably and unmolested in Athens, but Dionysius' last offence spurred him into action.
Dion formed a small army at Zacynthus, and returned to Sicily in 357 BC, much to the delight of the Syracusans. As Dionysius was in Caulonia, Italy at the time, Dion easily took all but Syracuse's island citadel. Dionysius sailed back to Syracuse immediately, and upon his return he attempted attacks from the citadel and tried to negotiate peace treaties. When he was unsuccessful in all attempts, he sailed away to Locri, Italy and left the citadel in the hands of his son Apollocrates.
In exile, Dionysius took advantage of the friendly citizens of Locri and became the city's tyrant, treating the locals with great cruelty. He did not return to Syracuse until 346 BC, eight years after Dion's assassination. Soon after he left Locri, the locals drove out the remaining troops and took their revenge on Dionysius' wife and daughters. Dionysius was able to regain power in Syracuse due only to its great political instability, as he was still very unpopular among the Syracusans. In the preceding several years, many other cities on Sicily had split from Syracuse to become ruled by more localized tyrants. Several of these cities joined the Syracusans in an attack against Dionysius, which proved to be quite successful and Dionysius was forced back into the citadel. At this time, 344 BC, Timoleon arrived and began his invasion of Sicily. Dionysius, out of respect for Timoleon and quite aware he no longer had a chance at victory, arranged the surrender of the citadel and was given safe passage to Corinth, Greece. For the next year until his death, Dionysius lived privately in Corinth in an increasingly miserable state.
Dionysius figures prominently in Mary Renault's historical novel The Mask of Apollo.