Chikuzen and Edo
He was born on 17 December 1813 (Bunka 10) in Hoshimaru, in Asakura (Chikuzen), today Fukuoka Prefecture, the second son of Itō Gozaemon; he took the surname Toyonaga from his maternal uncle. He claimed descent in the 39th generation from Samonji (左文字), the great swordsmith of Chikuzen. At the beginning of the Tenpō era he went to Edo and joined Shimizu Hisayoshi, a student of Hosokawa Masayoshi, in the line of Suishinshi Masahide. His first known dated work is from 1840 (Tenpō 11).
Tosa
In 1846 (Kōka 3) he went to Tosa with his wife. In the autumn of 1847 he met Sekida Katsuhiro, the domain swordsmith, and settled in his house in Mizudōri-chō, in Kōchi. He first signed Toyonaga Yukihide and, from 1848 (Kaei 1), Sa Yukihide. The record of his katana of 1851 in Bunka Isan Online says that he went down to Tosa on the recommendation of Sekida Katsuhiro, swordsmith to the Yamauchi.
Domain swordsmith
After Sekida’s death in 1855, on 14 June 1856 (Ansei 3) he succeeded him as domain swordsmith, also in charge of the iron tools workshop and of firearms. In March 1860 he returned to Edo and set up his forge at the secondary residence of Tosa in Sunamura, in Fukagawa, where he made swords and weapons.
The Bakumatsu years
According to the Japanese Wikipedia, in 1867 Inui Taisuke (乾退助), the future Itagaki Taisuke, hid rōnin from Mito in the Tosa residence, and Yukihide, who made firearms for the domain, denounced him with a copy of a letter as proof; Yamauchi Yōdō did not punish Inui. That September he was posted to Tosa, where he signed Tōko (東虎). Tani Tateki conjectured that he was in contact with the assassins of Sakamoto Ryōma and Nakaoka Shintarō; there is no proof.
Meiji and his death
His last dated work is from August 1870 (Meiji 3). It is usually explained that he stopped forging because of the ban on wearing swords; the Japanese Wikipedia believes that the supporters of the emperor treated him as a traitor and avoided his work. From 1872 he was protected by Itagaki Taisuke. He died on 5 March 1887 (Meiji 20), at the age of 75 by the Japanese count, in Ōsaka according to the Japanese Wikipedia; his grave is at the Reinen-ji temple, in Tennōji.
A katana of 1870 published by the NBTHK says on the tang that he made it at the age of 58 for his son Ikuma. Ikuma was called Toyonaga Ikuma (豊永幾馬), and Yukihide lived with him for a time in Yokohama.
Pupils
His students signed with the prefix Sa (左): Sa Hidetaka (左秀高), Hidechika (左秀近), Hideharu (左秀治), Hidekata (左秀方), Toshihide (左俊秀), Hidehiro (左秀弘), Ujitsura (左氏連) and Masanobu (左正宣).