He was the son of Yasui Shigetsugu, Lord of Miyago castle (Owari province), a descendant of
Hatakeyama Iekuni,
Shugo (Governor) of Kawachi province, descending from
Ashikaga Yoshikane (1154-1199). Yoshikane was the third son of
Minamoto no Yoshiyasu, also called Ashikaga Yoshiyasu (1127-1157), founder of the
Ashikaga clan, grandson of the
Chinjufu-shōgun (Commander-in-chief of the defense of the North)
Minamoto no Yoshiie (1039-1106), and a descendant of the
Emperor Seiwa (850-881), the 56th Emperor of Japan. He was adopted by his maternal uncle, Asano Nagakatsu, Lord of Asano castle, his mother's younger brother. He succeeded him as the fourteenth head of the Asano clan. Until Nagakatsu, the Asano descended directly from the
Toki clan and
Minamoto no Yorimitsu (948-1021). After Nagamasa, the Asano are direct descendants of the
Hatakeyama clan and the
Ashikaga clan. Since
O-Ne, who was born Sugihara Yasuko, daughter of Sugihara Sadatoshi, a descendant of Taira no Sadamori, and the
Emperor Kammu (735-806), and was adopted by her maternal uncle Asano Nagakatsu, a descendant of
Minamoto no Yorimitsu (944-1021), and the
Emperor Seiwa (850-881), married Kinoshita Tokichiro (later
Toyotomi Hideyoshi), despite her mother Asahidono opposition to this marriage because of the difference in social status with her husband, Nagamasa became a brother-in-law to Hideyoshi. In 1581, he fought in the Second
Tenshō Iga War under
Oda Nobunaga against inhabitants of Iga, at
Iga province. In 1582, Nagamasa also accompanied Hideyoshi in his campaign against the
Mōri clan, at
Siege of Takamatsu. In 1583, he distinguished himself in the
Battle of Shizugatake and was given 20,000 koku in Otsu (
Omi Province). In 1587, he served in the campaign to
suppress Kyushu and received
Obama Domain (80,000 koku) (
Wakasa province). In 1590, He fought for Hideyoshi against the
Hōjō clan at
Siege of Odawara, he captured
Iwatsuki and
Edo castles, also fought against the
Kunohe Rebellion in 1591. In 1593, made a great contribution to the dispatch of troops for the
Invasion of Korea and was given 215,000 koku of Fuchu (
Kai province). Asano was also appointed by Hideyoshi to a Commission of Five (
Go-Bugyō) along with
Ishida Mitsunari,
Maeda Gen'i,
Mashita Nagamori and
Natsuka Masaie. Asano held seniority over the Commissioners, who were charged with governing the capital of
Kyoto and the Home Provinces or
Kinai. A close advisor to Hideyoshi, Asano devised the land survey and a number of other policies enacted under his rule. In 1598, Hideyoshi's invasions were coming to an end, and Asano was sent to Korea with his fellow Commissioner Ishida Mitsunari to arrange for Japanese withdrawal. Asano was assured by the generals that the war was going well, and that they were on the verge of victory. Ishida disagreed, however, and supported withdrawal from Korea. Returning to Japan,
daimyō (feudal lords) from across the country became involved in the debate, and the disagreement grew into a major governmental rift. The
Go-Bugyō disbanded soon afterwards, having already been replaced by the
Council of Five Elders (
Tairō) by Hideyoshi before his death. ==Retired==