Alexander was the heir to his cousin, the 19-year-old Emperor who had been murdered along with his mother Julia Soaemias, by his own guards, who, as a mark of contempt, had their remains cast into the river Tiber. He and his cousin were both grandsons of the influential and powerful Julia Maesa, who had arranged for Elagabalus' acclamation as emperor by the famous Third Gallic Legion. It was the rumor of Alexander's death that triggered the assassination of Elagabalus and his mother. His 13-year reign was the longest reign of a sole emperor since Antoninus Pius. He was also the second-youngest ever sole legal Roman Emperor during the existence of the united empire, the youngest being Gordian III.
As emperor, Alexander's peace time reign was prosperous. However, Rome was militarily confronted with the rising Sassanid Empire and growing incursions from the tribes of Germania. He managed to check the threat of the Sassanids. But when campaigning against Germanic tribes, Alexander attempted to bring peace by engaging in diplomacy and bribery. This alienated many in the Roman Army and led to a conspiracy to assassinate and replace him. According to Canduci, Alexander is remembered as an emperor who was "level headed, well meaning, and conscientious", but his fatal flaw was his domination by his mother and grandmother. Not only did this undermine his authority, but his mother's influence was the cause of Alexander's least popular actions (convincing him not to take part in battle and trying to buy off the warring Germanic barbarians).
Born between 206-208, Severus Alexander became emperor when he was around 15 years old (though he could be 14 or 16), making him the youngest emperor in Rome's history, until the ascension of Gordian III. Alexander's grandmother believed that he had more potential to rule than her other grandson, the increasingly unpopular emperor Elagabalus, whom Alexander replaced. Thus, to preserve her own position, she had Elagabalus adopt the young Alexander and then arranged for Elagabalus' assassination, securing the throne for Alexander. The Roman army hailed Alexander as emperor on 13 March 222, immediately conferring on him the titles of Augustus, pater patriae and pontifex maximus.
Throughout his life, Alexander relied heavily on guidance from his grandmother, Maesa, and mother, Julia Mamaea. Maesa died in 223, leaving Mamaea as the sole influence upon Alexander's actions.
As a young, immature, and inexperienced adolescent, Alexander knew little about government, warcraft, or the role of ruling over an empire. Because of this, throughout his entire reign he was a puppet of his mother's advice and entirely under her jurisdiction, a state of affairs that was not popular with the soldiers.
Under the influence of his mother, Alexander did much to improve the morals and condition of the people, and to enhance the dignity of the state. He employed noted jurists to oversee the administration of justice, such as the famous jurist Ulpian. His advisers were men like the senator and historian Cassius Dio, and it is claimed that he created a select board of 16 senators, although this claim is disputed. He also created a municipal council of 14 who assisted the urban prefect in administering the affairs of the 14 districts of Rome. Excessive luxury and extravagance at the imperial court were diminished, and he restored the Baths of Nero in 227 or 229; consequently, they are sometimes also known as the Baths of Alexander after him.
Upon his accession he reduced the silver purity of the denarius from 46.5% to 43% — the actual silver weight dropped from 1.41 grams to 1.30 grams; however, in 229 he revalued the denarius, increasing the silver purity and weight to 45% and 1.46 grams. The following year he decreased the amount of base metal in the denarius while adding more silver, raising the silver purity and weight again to 50.5% and 1.50 grams. Additionally, during his reign taxes were lightened; literature, art and science were encouraged; and, for the convenience of the people, loan offices were instituted for lending money at a moderate rate of interest.