Michinaga exerted de facto reign over Japan in the early 11th century. This can be seen from the fact that he was father to four (non-reigning) empresses, uncle to two emperors and grandfather to another three.
He was the fourth or fifth son of Fujiwara no Kaneie by his wife Tokihime, daughter of Fujiwara no Nakamasa. There were two regents and two imperial consorts among his brothers and sisters by the same mother.
As the youngest son of his father, he was not remarkable in the court until his two brothers died. He started his career in the court when he was 15 years old. In 995 during the reign of Emperor Ichijō, his two elder brothers Michitaka and Michikane died of disease. He struggled with Fujiwara no Korechika, the elder son of Michitaka, for political power. With support of Senshi, his sister and mother of Ichijō, Michinaga succeeded in gaining power as well the support of majority of the court. He was appointed Nairan, the secretary of the emperor and reviewer who reviewed all the document before the emperor himself read them. Though he was not regent yet, he became then substantially the most powerful person in the court.
Though Ichijō already had an empress, a daughter of Michitaka, he claimed there were two types of empresshood and therefore it was legal for an emperor to have two empresses at the same time. Michinaga's ambitions led him make his own daughter, Shōshi, a second empress of Ichijō. In 1000 Shōshi was announced as a Chūgū empress and the existing empress Teishi was given the title of Kōgō empress. It was the first time an emperor had two empresses. (In 1006, Michinaga invited Murasaki Shikibu to become Empress Shōshi’s companion and tutor.) A power struggle between Korechika and Michinaga continued until Teishi's unexpected death, which sealed Michinaga's power since Shōshi became the only empress after Teishi's death. By Shōshi, two princes were born, later both crowned (Emperor Go-Ichijō and Emperor Go-Suzaku). Michinaga's other daughters, Kenshi and Ishi, followed similar fates to Shōshi and further ensured Michinaga's power over the court.
After Ichijō retired because of illness, Emperor Sanjō ascended the throne. Though Sanjō was a nephew of Michinaga (the mother of Sanjō was another sister of Michinaga; she had died already in Sanjō's childhood and he was relatively less influenced by his maternal line), Sanjō was already a mature man and had his own political view: he was older than his predecessor Ichijō and in his thirties when he became emperor.
Michinaga and Sanjō's opinions often varied. Michinaga pressured Sanjō to retire and finally Sanjō did so in 1016 under a condition made upon Sanjō's succession. Sanjō's elder son was appointed as Go-Ichijō's successor, but Michinaga's political power and influence led to the crown prince's resignation by his will. Michinaga was pleased by this decision and gave his daughter (either Kenshi or Ishi) to this prince as a wife, ensuring that the prince would not be an obstacle in the future.
Technically, Michinaga never formally took on the title of kampaku regent, but in reality his word was law, even after he formally retired from public life in 1019. He continued to direct the affairs of his son and successor, Yorimichi. Michinaga is popularly known as the Mido Kampaku, implying that he had usurped the full power of a kampaku without necessarily calling himself that, though he retained the title sesshō regent in a short term from 1016 till 1017. In 1017, he gave this office to his heir Yorimichi.
Soon afterwards, a series of emperors started to retire to a monastery early in life, and put their young sons on the throne to run the country from behind the scenes. As it turned out, this tactic briefly allowed the emperors to wrestle power back from the Fujiwara clan, only to see it fall to the Taira warrior clan instead.
"This world, I think,
Is indeed my world.
Like the full moon I shine,
Uncovered by any cloud."
This poem is known as Mochizuki no Uta (望月の歌) (Full moon Poem). In 1018, his daughter Ishi became Empress (Chūgū) of Emperor Go-Ichijō, Kenshi became Empress Dowager (Kōtaigō), and Shōshi was Grand Empress Dowager (Tai-Kōtaigō). Three of his daughters became Empresses. Furthermore both Emperor Go-Ichijō and Crown Prince Atsunaga were his grandsons. This poem was composed at the party to celebrate his daughter's accession. He expressed his delight to win power in this poem.
He was also married to Minamoto no Meishi (源明子), daughter of Sadaijin Minamoto no Takaakira. They had six children.
Michinaga had one daughter from unknown woman.