In 1553, at seven years old, he was sent to the Takeda clan's headquarters in Kai as a hostage.
There he becomes part of the Okukinjūshū (奥近習衆), a group of six young servants close to Takeda Shingen.
According to the Kōyō Gunkan, Shingen favoured him as he soon recognized that Masayuki's talents and insight rivaled those of his father Yukitaka.
As such, he is sometimes included among the Twenty Four Generals, alongside his father and two older brothers.

In 1558, he became the foster son of the Mutō family, a branch of the Ōi clan, of which Shingen's mother descended from, and adopted the name Mutō Kihei (武藤喜兵衛).
 Towards 1564, he married Yamanote-dono (山手殿), a daughter of Uda Yoritada, who was a local lord of Tōtōmi Province.
Later she gave birth to his two sons Nobuyuki and Nobushige.
 During this period, he participated in many battles under the Takeda clan, including the Fourth Battle of Kawanakajima (1561) and the Battle of Mimasetōge (1569).
Also most importantly, from 1572 onwards, he joined Shingen in his campaign towards Kyoto against the Oda and Tokugawa clans and took part in the Battle of Mikatagahara (1573).

In May 1573, Shingen died amidst his campaign and so Masayuki continued to serve his heir Takeda Katsuyori.

In 1574, his father Yukitaka died.
At that point, his eldest brother Sanada Nobutsuna had already succeeded his father as the head of the Sanada clan.
However, during the disastrous Battle of Nagashino (1575) against the Oda clan, both his older brothers, Nobutsuna and Masateru, were killed, so he came back to Sanada clan and claimed his inheritance.
In this, Masayuki supposedly had the support of Kōsaka Masanobu, who held Kaizu Castle in Northern Shinano and was also a chief retainer of the Takeda clan.
Katsuyori accepted his claim without any qualms.

In 1579, a year after Uesugi Kenshin's death, an alliance between the Takeda and Uesugi clans was established.
The following year, ordered by Takeda Katsuyori, Masayuki invaded western Kōzuke, which was a Hojō domain at the time, and seized Numata Castle, putting it under control of the Takeda clan.
The same year, he was appointed the title of Awa-no-kami (従五位下・安房守).

In 1581, he was ordered by Katsuyori to supervise the construction of the new Shinpu Castle at Nirasaki.
In the same year, Numata Kageyoshi, former lord of Numata Castle, attempted to retake his old fief, but Masayuki schemed to assassinate him and thwarted his plans.

In April 1582, Oda and Tokugawa allied forces started an invasion of the Takeda territory.
It is said that Masayuki had intended to shelter Katsuyori and advised him to abandon Kai Province and flee towards Sanada's domain in Kōzuke(Iwabitsu Castle).
Instead, Katsuyori decided to take shelter at Oyamada Nobushige's Iwadono Castle, but was betrayed and ultimately died at Tenmokuzan.
After the fall of the Takeda clan, Masayuki yielded to Oda Nobunaga and was put under the orders of one of Nobunaga's chief commanders, Takigawa Kazumasu.
Masayuki managed to retain most of his domain, but had to abdicate Numata Castle to Takigawa Masushigue, Kazumasu's relative.
In 1553, at seven years old, he was sent to the Takeda clan's headquarters in Kai as a hostage. There he becomes part of the Okukinjūshū (奥近習衆), a group of six young servants close to Takeda Shingen. According to the Kōyō Gunkan, Shingen favoured him as he soon recognized that Masayuki's talents and insight rivaled those of his father Yukitaka. As such, he is sometimes included among the Twenty Four Generals, alongside his father and two older brothers.
In 1558, he became the foster son of the Mutō family, a branch of the Ōi clan, of which Shingen's mother descended from, and adopted the name Mutō Kihei (武藤喜兵衛).  Towards 1564, he married Yamanote-dono (山手殿), a daughter of Uda Yoritada, who was a local lord of Tōtōmi Province. Later she gave birth to his two sons Nobuyuki and Nobushige.  During this period, he participated in many battles under the Takeda clan, including the Fourth Battle of Kawanakajima (1561) and the Battle of Mimasetōge (1569). Also most importantly, from 1572 onwards, he joined Shingen in his campaign towards Kyoto against the Oda and Tokugawa clans and took part in the Battle of Mikatagahara (1573). 
In May 1573, Shingen died amidst his campaign and so Masayuki continued to serve his heir Takeda Katsuyori.
In 1574, his father Yukitaka died. At that point, his eldest brother Sanada Nobutsuna had already succeeded his father as the head of the Sanada clan. However, during the disastrous Battle of Nagashino (1575) against the Oda clan, both his older brothers, Nobutsuna and Masateru, were killed, so he came back to Sanada clan and claimed his inheritance. In this, Masayuki supposedly had the support of Kōsaka Masanobu, who held Kaizu Castle in Northern Shinano and was also a chief retainer of the Takeda clan. Katsuyori accepted his claim without any qualms. 
In 1579, a year after Uesugi Kenshin's death, an alliance between the Takeda and Uesugi clans was established. The following year, ordered by Takeda Katsuyori, Masayuki invaded western Kōzuke, which was a Hojō domain at the time, and seized Numata Castle, putting it under control of the Takeda clan. The same year, he was appointed the title of Awa-no-kami (従五位下・安房守).
In 1581, he was ordered by Katsuyori to supervise the construction of the new Shinpu Castle at Nirasaki. In the same year, Numata Kageyoshi, former lord of Numata Castle, attempted to retake his old fief, but Masayuki schemed to assassinate him and thwarted his plans.
In April 1582, Oda and Tokugawa allied forces started an invasion of the Takeda territory. It is said that Masayuki had intended to shelter Katsuyori and advised him to abandon Kai Province and flee towards Sanada's domain in Kōzuke(Iwabitsu Castle). Instead, Katsuyori decided to take shelter at Oyamada Nobushige's Iwadono Castle, but was betrayed and ultimately died at Tenmokuzan. After the fall of the Takeda clan, Masayuki yielded to Oda Nobunaga and was put under the orders of one of Nobunaga's chief commanders, Takigawa Kazumasu. Masayuki managed to retain most of his domain, but had to abdicate Numata Castle to Takigawa Masushigue, Kazumasu's relative.
Sanada Masayuki