In 1583, he assisted his father at the Battle of Shizugatake and after the death of Shibata Katsuie, he swore fealty to Toyotomi Hideyoshi and was awarded a 40,000 koku fief in Kaga Province.

In 1585, he played a key role in Hideyoshi's campaign against Sassa Narimasa, and his holdings were greatly expanded to 320,000 koku across Kaga and Etchū Provinces.

Toshiie died the following year 1599, and Hosokawa Tadaoki convinced Toshinaga to support Tokugawa Ieyasu over Ishida Mitsunari and Toyotomi Hideyori.

In 1600, at the time of the Battle of Sekigahara, his role was primarily to contain the forces of Uesugi Kagekatsu and Niwa Nagashige, they clash at Battle of Asainawate, and at the same time, to keep the forces of his younger brother, Maeda Toshimasa, ruler of Noto Province, from joining the western army.

During this period, he increasingly withdrew from public life due to complications from syphilis, and committed suicide by poison at Takaoka Castle in 1614.