Masanaga was a swordsmith of the Osafune school in Bizen Province, active during the late Kamakura period. He is recorded as a student of Nagamitsu, and his dated works span from the Shoan era through Kagen and Enkyo, with a signed tachi bearing the date Enkyo 3 (1310). Most of his signatures appear as the two-character mei "Masanaga," though examples with a long signature reading "Bizen no Kuni Osafune Junin Masanaga Tsukuru" are also known. It is said that he served as one of the daisaku (substitute makers) for his master Nagamitsu, and he was active in the same period as Kagemitsu.

The forging of Masanaga's blades is characteristically a tight ko-itame-hada, well consolidated and showing magnificent midare-utsuri standing clearly in the steel. The hamon favors suguha -- typically a nioi-deki suguha with tight nioiguchi, small ashi, and on occasion slight gunome mixed in. The sugata of his tachi retain the classic late Kamakura proportions, with koshi-zori and chu-kissaki. The jigane and tempering are consistently praised as excellent, and the clarity of the utsuri is a distinguishing feature that links his work firmly to the Osafune mainstream tradition of the Nagamitsu lineage.

Among Masanaga's surviving works, signed and dated examples are of considerable documentary value. He has a relatively substantial body of extant work compared to many of his contemporaries, and works such as the tachi in the Imperial Collection demonstrate his adherence to the refined Osafune idiom at its height. One blade bearing the date Shochu 3 (1326) has been singled out as particularly valuable for its documentary significance. His workmanship has been assessed as superior to that of contemporaries such as Kageyori and Yasuhiro, reflecting a smith who faithfully maintained the high standards of the Osafune school during its most distinguished era.