Eucratides came to the throne by overthrowing the dynasty of Euthydemus I in Bactria, whose son Demetrius was conquering northwestern India. The king whom Eucratides dethroned in Bactria was probably Antimachus I.
It is unclear whether Eucratides was a Bactrian official who raised a rebellion, or, according to some scholars, a cousin of the Seleucid king Antiochus IV Epiphanes who was trying to regain the Bactrian territory. Justin explains that Eucratides acceded to the throne at about the same time as Mithridates, whose rule is accurately known to have started in 171 BC, thereby giving an approximate date for the accession of Eucratides:
"Around the same time, two great men started to rule: Mithridates among the Parthians, and Eucratides among the Bactrians" Justin XLI,6
Some of the coins of Eucratides probably represent his parents, where his father is named Heliocles, and his mother, who is thought to be Laodice, is wearing a royal diadem. Laodice may have been a member of the Seleucid imperial house.
Having become master of Bactria, Eucratides also conquered the western parts of the Indo-Greek kingdom. According to the single remaining source, Roman historian Justin, Eucratides defeated Demetrius of India, but the identity of this king is uncertain: he could be either Demetrius I, or Demetrius II.
"Eucratides led many wars with great courage, and, while weakened by them, was put under siege by Demetrius, king of the Indians. He made numerous sorties, and managed to vanquish 60,000 enemies with 300 soldiers, and thus liberated after four months, he put India under his rule" Justin XLI,6
Numismatic evidence suggests that Eucratides I was a contemporary of the Indo-Greek kings Apollodotus I, Antimachus II and Menander I. In any case, Eucratides' advances into India are proved by his abundant bilingual coinage.
In the west the Parthian king Mithradates I began to enlarge his kingdom and attacked Eucratides; the city of Herat fell in 167 BC and the Parthians succeeded in conquering two provinces between Bactria and Parthia, called by Strabo the country of Aspiones and Turiua.
Eucratides I is most likely the founder of Eucratideia.
The seal of Da Afghanistan Bank features a Eucratides I-era coin.
Justin ends his account of Eucratides' life by claiming that the warlike king was murdered on his way back from India by his own son (either Eucratides II or Heliocles I, although there are speculations that it could be his enemy's son Demetrius II), who hated his father so much that he dragged his dead body after his chariot:
"As Eucratides returned from India, he was killed on the way back by his son, whom he had associated to his rule, and who, without hiding his patricide, as if he didn't kill a father but an enemy, ran with his chariot over the blood of his father, and ordered the corpse to be left without a sepulture" Justin XLI,6
The murder of Eucratides probably brought about a civil war amongst the members of the dynasty. The successors to Eucratides were Eucratides II and Heliocles I (145–130 BC), who was the last Greek king to reign in Bactria. Once the Yuezhi tribes overpowered Heliocles, the Greco-Bactrians lost control of the provinces north of the Hindu Kush.
Two other members of the dynasty were Plato of Bactria and probably Demetrius II, who in that case was not identical with the king Justin claimed was the enemy of Eucratides I.
The rule of the Greco-Bactrians soon crumbled following these numerous wars:
"The Bactrians, involved in various wars, lost not only their rule but also their freedom, as, exhausted by their wars against the Sogdians, the Arachotes, the Dranges, the Arians and the Indians, they were finally crushed, as if drawn of all their blood, by an enemy weaker than them, the Parthians." Justin, XLI,6
However, the rule of the Indo-Greeks over territories south of the Hindu Kush lasted for a further 150 years, ultimately collapsing under the pressure of the Yüeh-chih and Scythian (Saka) invasions in around 10 BC, with the last Indo-Greek ruler Strato II.