Masanari was a son of Hayashi Masahide, and his mother was of the Andô clan. He married his second wife, Fuku (later to be known as Kasuga no Tsubone) in 1595, after the death of his first wife, a daughter of Saitô Toshimitsu, and older sister to Fuku. He was first a vassal of Toyotomi Hideyoshi, and after his death, he became a vassal and advisor to Kobayakawa Hideaki. Before the Battle of Sekigahara he counceled Hideaki to defect to the Eastern army, led by Tokugawa Ieyasu. On the night before the battle, Ôtani Yoshitsugu met with Masanari and Hideaki, and, perhaps to keep up appearances, the former gave his word to Yoshitsugu that Hideaki would fight on the side of the Western army. During the battle, Hideaki did indeed defect to the Tokugawa side, possibly winning the battle for Ieyasu.
Shortly before Hideaki's death in 1602, Masanari left the service of his lord and retired to Mino province, possibly in order to escape any "responsibility" for Hideaki's treason at Sekigahara.
Masanari died on the 9th month, 17th day of 1628.
- It is possible that his wife Fuku, who must have hated Hideyoshi for the murder of her father, may have had a hand in convincing Masanari to advise Hideaki to turn against the Eastern army, which represented Hideyoshi
- Some sources state that he was forced into retirement by the death of his lord
- Sengoku Jinmei Jiten
- Dykstra, Yoshiko. The Shôgun's Woman: Lady Kasuga University of Hawaii, Center for Japanese Studies lecture. Honolulu, Hawaii. 9 November, 2006.