He was born at Naissus in Upper Dacia to Constantius I Chlorus and an innkeeper's daughter, Helena. Constantine was well educated and served at the court of Diocletian after the appointment of his father as one of the two Caesari, at that time a junior emperor, in the Tetrarchy in 293. On the death of his father Constantius in 306, he managed to be at his deathbed in Eburacum (York), where troops loyal to his father's memory proclaimed him Emperor. For the next 18 years he fought a series of battles and wars that left him as supreme ruler of the Roman Empire.
Constantine is perhaps best known for being the first Roman Emperor to endorse Christianity, as a result of his victory in the Battle of Milvian Bridge, which he credited to the Christian God. His adoption of Christianity may also be due to family influence: Helena was probably born a Christian, and demonstrated extreme piety in her later life. He legalized and strongly supported Christianity beginning around the time he became emperor, with the Edict of Milan, but he neither made paganism illegal nor made Christianity the state religion. Though the church prospered under Constantine's patronage, it also fell into the first of many public schisms. He himself called the First Council of Nicaea to settle the problem of Arianism, a dispute about the personhood and godhood of Jesus. He himself was not baptized and chrismated until close to his death. Ironically, Constantine may have favored the losing side of the Arian controversy, as he was baptized by an Arian bishop, Eusebius of Nicomedia.
His victory in 312 over Maxentius at the Battle of Milvian Bridge resulted with him becoming Western Augustus, or ruler of the entire western half of the empire. He gradually consolidated his military superiority over his rivals in the crumbling Tetrarchy until 324, when he defeated the eastern ruler, Licinius, and became sole emperor.
Constantine also rebuilt the ancient Greek city of Byzantium, naming it Nova Roma, providing it with a Senate and civic offices similar to the older Rome. After his death it was renamed Constantinople, and gradually became the capital of the empire.
Although he earned his honorific of "The Great" from Christian historians long after he had died, he could have claimed the title on his military achievements alone. In addition to reuniting the empire under one emperor, Constantine also won major victories over the Marcomanni and Alamanni (306-08), the Vandals and Marcomanni (314-15), the Visigoths in 332 and the Sarmatians two years later. In fact, by 336, Constantine had actually reoccupied most of the long-lost province of Dacia, which Aurelian had been forced to abandon in 271. At the time of his death, he was planning a great expedition to put an end to raids on the eastern provinces from Persia by conquering that nation--something no Emperor since Trajan had contemplated.
He was succeeded by his three sons, Constantine II, Constantius II and Constans, who secured their hold on the empire with the murder of a number of relatives and supporters of Constantine. The last member of his dynasty was his grandson, Julian, who attempted to restore paganism.
Constantius Chlorus (305-306),
Galerius (305 - 311)
Constantius II (337-361),
Constantine II (337 - 340),
and Constans (337 - 350)