Born in 1527 as the son of Kagawa Motokage, head of the Sanuki Kagawa clan descended from the Yoshishige line of the Kanmu Taira. He ruled Sanuki's Tado, Mino, and Toyota districts (later adding Naka District), based at Hondaiyama Castle (Tadotsu Castle), his clan's peacetime residence, with Amagiri Castle as its fortress of last resort. Initially serving the Sanuki constable Hosokawa clan, he resisted the rising Miyoshi clan as Hosokawa authority waned. In 1576, he pledged allegiance to Oda Nobunaga and received a character from his name, changing his own from 'Yukikage' to 'Nobukage,' maintaining independence with Mori support as well. But around 1578, faced with Chosokabe Motochika's advance to unify Shikoku, he chose to submit — adopting Motochika's second son Chikakazu as his son-in-law and heir to seal the peace. After Motochika's defeat in Hideyoshi's 1585 conquest of Shikoku, the Kagawa house too was dissolved, ending over 200 years of Sanuki Kagawa rule. Nobukage himself fled to Tosa and died there in August 1600; little is recorded of his final years.

A pragmatist who navigated Shikoku's ever-shifting balance of power among the Hosokawa, Miyoshi, Oda, Mori, and Chosokabe, using flexible diplomacy — including the adoption alliance — to try to preserve his 200-year-old house. In the end, even his skill could not save the clan from the irresistible tide of Hideyoshi's conquest of Shikoku.

The ruins of Hondaiyama Castle lie within present-day Torizuka Park in Tadotsu, now known as a cherry-blossom viewing spot. The Kagawa clan's adoption of Chosokabe Motochika's son is often cited as a textbook example of Motochika's strategy of expanding influence through marriage and adoption alliances across Shikoku.