During Simeon's rule, Bulgaria spread over a territory between the Aegean, the Adriatic and the Black Sea. The newly independent Bulgarian Orthodox Church became the first new patriarchate besides the Pentarchy, and Bulgarian Glagolitic and Cyrillic translations of Christian texts spread all over the Slavic world of the time. It was at the Preslav Literary School in the 890s that the Cyrillic alphabet was developed. Halfway through his reign, Simeon assumed the title of Emperor (Tsar), having prior to that been styled Prince (Knyaz).
Simeon was born in 864 or 865, as the third son of Knyaz Boris I of Krum's dynasty. As Boris was the ruler who Christianized Bulgaria in 865, Simeon was a Christian all his life. Because his eldest brother Vladimir was designated heir to the Bulgarian throne, Boris intended Simeon to become a high-ranking cleric, possibly Bulgarian archbishop, and sent him to the leading University of Constantinople to receive theological education when he was thirteen or fourteen. He took the name Simeon as a novice in a monastery in Constantinople. During the decade (ca. 878–888) he spent in the Byzantine capital, he received excellent education and studied the rhetoric of Demosthenes and Aristotle. He also learned fluent Greek, to the extent that he was referred to as "the half-Greek" in Byzantine chronicles. He is speculated to have been tutored by Patriarch Photios I of Constantinople, but this is not supported by any source.
Around 888, Simeon returned to Bulgaria and settled at the newly established royal monastery of Preslav "at the mouth of the Tiča", where, under the guidance of Naum of Preslav, he engaged in active translation of important religious works from Greek to Old Church Slavonic (Old Bulgarian), aided by other students from Constantinople. Meanwhile, Vladimir had succeeded Boris, who had retreated to a monastery, as ruler of Bulgaria. Vladimir attempted to reintroduce paganism in the empire and possibly signed an anti-Byzantine pact with Arnulf of Carinthia, forcing Boris to re-enter political life. Boris had Vladimir imprisoned and blinded, and then appointed Simeon as the new ruler. This was done at an assembly in Preslav which also proclaimed Bulgarian as the only language of state and church and moved the Bulgarian capital from Pliska to Preslav, to better cement the recent conversion. It is not known why Boris did not place his second son, Gavril, on the throne, but instead preferred Simeon.
With Simeon on the throne, the long-lasting peace with the Byzantine Empire established by his father was about to end. A conflict arose when Byzantine Emperor Leo VI the Wise, allegedly acting under pressure from his mistress Zoe Zaoutzaina and her father Stylianos Zaoutzes, moved the marketplace for Bulgarian goods from Constantinople to Thessaloniki, where the Bulgarian merchants were heavily taxed. The Bulgarians sought protection by Simeon, who in turn complained to Leo. However, the Byzantine emperor ignored his embassy.
Forced to take action, in the autumn of 894 Simeon invaded the Byzantine Empire from the north, meeting with little opposition due to the concentration of most Byzantine forces in eastern Anatolia to counter Arab invasions. Informed of the Bulgarian offensive, the surprised Leo sent an army consisting of guardsmen and other military units from the capital to halt Simeon, but his troops were routed somewhere in the theme of Macedonia. The Bulgarians took most of the Khazar mercenary guardsmen prisoners and killed many archons, including the army's commander. However, instead of continuing his advance to the Byzantine capital, Simeon quickly withdrew his troops to face a Magyar invasion from the north. These events were later called "the first trade war in medieval Europe" by Bulgarian historians.