Toshimasu served under Oda Nobunaga along with his uncle. Toshimasu was originally intended to inherit Maeda family headship; however, after Oda Nobunaga replaced Toshihisa with Toshiie as Maeda family head, he lost this position. Perhaps because of this loss of inheritance, Toshimasu is well known for not getting along with his uncle.

While in Kyoto, Toshimasu met and befriended Naoe Kanetsugu, Uesugi Kagekatsu's kar?. The two became close friends. Consequently, Toshimasu agreed to join Kanetsugu in the Uesugi clan's invasion of Aizu. During the retreat from the failed invasion, Keiji was given the task of leading the rear guard. Riding his horse Matsukaze into battle and brandishing a two-pronged spear, he made a splendid show of force. Due in part to Toshimasu's actions, the Uesugi forces were able to retreat largely intact.

After this, Toshimasu returned to the capital and devoted himself to arts and literature. Keiji was barred from Toyotomi’s Kyushu campaign for his wild way. When the Tokugawa challenged the Uesugi in 1600, he once again fought with Uesugi’s army. In the battle against the Mogami, he broke through the enemy lines with only eight riders, and shattered their formation. After the Uesugi clan's move to the Yonezawa Domain, Toshimasu remained with them, serving as a retainer.

Keiji's armour can still be seen today at the Miyasaka Museum.

Matsukaze, which means "wind in the pines", was Toshimasu's famous mount. According to the legend, he was born through selective breeding of only the finest horses available, but unlike other horses he refused to let anyone ride him and eventually ran away. By unknown methods, Toshimasu managed to tame the wild horse; some have attributed it due to Toshimasu's wild personality. Since then, the two were never seen apart.

Matsukaze was said to be a horse of immense strength, able to carry his master's large frame for days. After his master's death, it is said that Matsukaze ran off and was never seen again.