Hoshina Masayuki was born in Edo, the illegitimate 4th son of the 2nd shōgun, Tokugawa Hidetada. As Masayuki's mother, Oshizu no Kata was a servant, Hidetada chose to hide the newborn, then named Komatsu. This was to protect him from potential infanticide at the hands of Oeyo, Hidetada's wife. Komatsu was later secretly given in adoption to Hoshina Masamitsu, a former Takeda retainer, and daimyō of Takatō Domain. In 1629, Komatsu first met his elder brother, the third Shōgun Tokugawa Iemitsu, father the retired-Shōgun and another older brother Tokugawa Tadanaga, and was given some mementos of his grandfather, Tokugawa Ieyasu as recognition of his status as a member of the Tokugawa lineage. In 1631, Komatsu inherited the Hoshina clan chieftainship, as well as the Takatō fief, and changed his name to "Masayuki". He was also granted the courtesy titles of Higo-no-kami and Sakonoegonchūjō and court rank of Upper 4th, Junior grade. Following the death of Tokugawa Hidetada, Masayuki became one of the closest confidants and assistants to Tokugawa Iemitsu, and wielded political power considerably greater than his official status of a 30,000 kokudaimyō would indicate. He was ordered to construct his Edo resident immediately outside the Sakurada Gate to Edo Castle, was entrusted with the construction of Hidetada's mausoleum at Zōjō-ji and with the 17thmemorial services for Tokugawa Ieyasu at Nikkō Tōshō-gū. He was instrumental in writing the Buke shohatto regulations issued in 1635, which codified the laws and regulations governing the conduct of the daimyō and samurai under the shogunate. In 1636, Masayuki was transferred to Yamagata Domain, which a kokudaka of 200,000 koku, which was a very considerable increase in income. During the Shimabara rebellion of 1637, he was made general of reinforcements which the shogunate had prepared to send to Kyushu, but the rebellion was crushed before the army could be dispatched. In 1643, he was again transferred, this time to Aizu Domain in Mutsu Province, where his kokudaka was officially set at 230,000 koku; however, as Aizu Domain was also entrusted with the administration of all tenryō territories in the Aizu area, his true income was more than double this amount. Masayuki was entrusted by Tokugawa Iemitsu with the regency during the minority of his nephew, the 4th Shōgun Tokugawa Ietsuna, which made him virtual ruler of Japan for several years until Ietsuna reached his majority. However, he refused an offer to be allowed use the Tokugawa family crest and to change his surname to Matsudaira, partly out of respect for the Hoshina clan which raised him, and partly to emphasize that he had no designs to usurp the authority of the main line of the Tokugawa lineage.. Masayuki was also a patron of Yamazaki Ansai, one of the early figures in Edo-era Japanese Neo-Confucianism, and together with him wrote the famous Aizu House Code, which included a direct injunction regarding the loyalty of the clan to the Shōgun. In 1669, he officially retired from his duties, turning over Aizu Domain to his son, Hoshina Masatsune. He died at the clan's residence in Mita in Edo in 1672. Masakuni was a disciple of Yoshikawa Shintō and per his request, he was buried per Shinto rituals on the grounds of Hanitsu Shrine near Lake Inawashiro, and was deified after his death as the kamiHanitsu-reishin. Masayuki's descendants remained enfeoffed at Aizu Domain for the remainder of the Edo period, until they were defeated by the forces of the new Meiji government during the Boshin War.
Tokugawa Iemitsu asked the famed swordsman Miyamoto Musashi to paint a screen portraying wild ducks. This was to pass into the hands of Masayuki, who took it with him to Aizu, and kept it as one of his family treasures.
