Stephen was the second son of Romanos I and his wife
Theodora. His older siblings were
Christopher (co-emperor from 921 until his death in 931) and his sisters
Helena, who married
Constantine VII Porphyrogennetos (r. 913–959), and Agatha, who married Romanos Argyros. His younger brothers were
Constantine (co-emperor 924–945) and
Patriarch of Constantinople in 933–956). He probably also had at least two unnamed younger sisters, known only because of their marriages to the
magistroi Romanos Mosele and Romanos Saronites.
Romanos Lekapenos had risen to power in 919, when he had managed to appoint himself regent over the young Constantine VII and marry his daughter Helena to him. Within a year, he successively rose from
Caesar, and was eventually crowned senior emperor on 17 December 920.
 To consolidate his hold on power, and with a view of supplanting the ruling
Macedonian dynasty with his own family, he raised his eldest son Christopher to co-emperor in May 921, while Stephen and Constantine were proclaimed co-emperors on 25 December 924.
Following Christopher's early death in 931, and given Constantine VII's de facto sidelining, Stephen and Constantine assumed an increased prominence, although formally they still ranked after their brother-in-law in the college of emperors. In 933, Stephen was married to Anna, the daughter of a certain Gabalas, who was crowned
Augusta on the same occasion.
 The couple had one known son, Romanos. According to the 11th-century chronicler
George Kedrenos, he was
castrated in 945, but later became a
Stephen and Constantine Lekapenos came to the fore in 943, when they opposed a dynastic marriage for their nephew,
Romanos II. Their father wanted to have his eldest surviving grandson married to Euphrosyne, a daughter of his successful general
John Kourkouas. Although such a union would effectively cement the loyalty of the army, it would also strengthen the position of the legitimate
Macedonian line, represented by Romanos II and his father Constantine VII, over the imperial claims of Romanos's own sons.
 Predictably, Stephen and Constantine opposed this decision, and prevailed upon their father, who was by this time ill and old, to dismiss Kourkouas in the autumn of 944.
 Romanos II instead married Bertha, an illegitimate daughter of
Hugh of Arles,
King of Italy, who changed her name to Eudokia after her marriage.
With Romanos I approaching the end of his life, the matter of his succession became urgent. In 943, Romanos drafted a
will which would leave Constantine VII as the senior emperor following his death. This greatly upset his two sons, who feared that their brother-in-law would have them deposed and force them to take monastic vows. Motivated, in the opinion of
Steven Runciman, partially by self-preservation and partially from genuine ambition, they started planning to seize power through a
coup d'état, with Stephen apparently the ringleader and Constantine a rather reluctant partner.
Their fellow conspirators included
Marianos Argyros, the
Manuel Kourtikes, the
strategos Diogenes, Clado, and Philip. Kedrenos, however, considers Peteinos to have served as an agent of Constantine VII among the conspirators. On 20 December 944, the conspirators set their plans in motion. The two brothers smuggled their supporters in the
Great Palace of Constantinople during the midday break in palace activities. They then led their men into the chamber of Romanos I, where they easily captured the "ill old man". They were able to transport him to the nearest harbour and from there to
Prote, one of the
Princes' Islands and a popular place of exile. There, Romanos agreed to take monastic vows and retire from the throne.
Having managed to quietly depose their father, the brothers now had to deal with Constantine VII. Unfortunately for them, rumours soon spread around
Constantinople, to the effect that, following Romanos's deposition, Constantine VII's life was in danger. Before long, crowds gathered before the palace, demanding to see their emperor in person. The contemporary
Liutprand of Cremona notes that the ambassadors and envoys from
Provence present in the capital also supported Constantine VII. Stephen and his brother had to succumb to the inevitable, recognizing their brother-in-law as the senior emperor.
The new triumvirate lasted for about 40 days. The three emperors soon appointed new leaders for the military services.
Bardas Phokas the Elder was appointed as the new
Domestic of the Schools, and
Constantine Gongyles as head of the
Byzantine navy. Stephen and his brother managed to reward their fellow conspirators. Peteinos became
Great Hetaeriarch, Argyros was appointed
Count of the Stable, Kourtikes a patrikios and
droungarios of the Watch.
 On 26 January 945, however, at the urging of their sister, the Augusta Helena, another coup removed the two Lekapenoi from power and restored the sole imperial authority to Constantine VII.
Initially, the two brothers were sent to Prote. The Byzantine chroniclers have their father welcoming them by quoting a passage from the
Book of Isaiah, specifically Chapter 1.2:
 "Hear, O heavens, and give ear, O earth; for
Jehovah hath spoken: I have nourished and brought up children, and they have rebelled against me."
 Liutprand of Cremona, however, gives a slightly different account, having Romanos receive his sons with bitter sarcasm, thanking them for not neglecting him and begging them to excuse the monks for their ignorance on how to properly receive emperors. Soon, however, Stephen was moved on to a prison at
Prokonnesos, and then to
Rhodes, before finally settling in
A plot by some members of the imperial government to restore him was discovered in December 947 and the conspirators
mutilated and publicly humiliated. Stephen died at Methymna on
Easter Sunday, 963.
John Skylitzes claims that Stephen was poisoned by order of the Empress
Theophano as part of her efforts to protect the rights to the throne of her sons
Basil II and
Constantine VIII, by eliminating other possible claimants to the throne. It should, however, be noted that several deaths of the extended imperial family at the time are attributed to Theophano by hostile sources, usually by poison.