Michiyo Yagi grew up in Tokoname City, in Aichi Prefecture, with a koto in the house. Her mother taught the instrument locally and put her in front of one at around four years old. She did not take to it. Japanese schooling in that period treated Japanese music as the lesser thing beside Western music, and Yagi went to the piano instead, studying it seriously from the age of five. The koto stayed her mother's instrument for most of a decade.

What changed her mind was a performance. In high school she heard Teiko Kikuchi play a contemporary composition on the 17-string koto, and the instrument stopped sounding like inherited furniture and started sounding like something with a future in it. She began studying seriously with Satomi Kurauchi, herself an apprentice of Tadao Sawai. Yagi graduated from Aichi Junior College in 1983, and two years later was admitted to the advanced studies programme at the Sawai Koto School, where she lived in the home of Tadao and Kazue Sawai as a full-time apprentice. She also came through the NHK Professional Training School for Traditional Musicians. By the second half of the 1980s she was performing professionally.

The turn toward improvisation happened in the United States. In 1989 she appeared at the Bang on a Can festival in New York alongside Kazue Sawai and encountered John Cage's work at close range. In the early 1990s she took a post as guest instructor in the ethnomusicology department at Wesleyan University in Connecticut, the department with which Cage had long been associated, and premiered commissioned pieces by Christian Wolff and John Zorn. She had crossed the Pacific as a specialist in a traditional instrument. She returned as an improviser, and she has spent every decade since declining to be filed under either heading. Asked to describe the musician she wants to be, she has answered: "a genre unto myself."

Her first records came through the groups. Koto Vortex, a quartet of fellow Sawai apprentices with Yoko Nishi, Miki Maruta and Etsuko Takezawa, released two albums in 1994 and 1996. Kokoo, a trio she co-founded, plays original material and progressive rock covers on traditional Japanese instruments only, and put out Super-Nova on Seven Seas in 2000. Hoahio, the avant-pop trio she forms one third of, landed Ohayo! Hoahio! (2000) and Peek-Ara-Boo (2003) on Zorn's Tzadik label.

Her own name arrived on Tzadik first. Shizuku (TZ 7218, 1999) is a solo koto record produced by Zorn, with cover design by Ikue Mori and photography by Nobuyoshi Araki. Seventeen followed in 2005 on Zipangu Products, built entirely on the 17-string bass koto, the low end of the family and the size that lets her hold a bottom register rather than decorate someone else's.

From 2006 she ran her own imprint, Idiolect, and it documents the working partnership that has shaped her sound more than any other. Norwegian drummer Paal Nilssen-Love plays on Live! At Super Deluxe (2006, with bassist Ingebrigt Håker Flaten), on Head On with Peter Brötzmann (2008), on Volda with Brötzmann again (2010), on Angular Mass with Lasse Marhaug (2015), on Soul Stream with Joe McPhee and Marhaug (2015), and on Decayed, recorded live at Aketa no Mise in Tokyo with Håker Flaten and guest guitarist Henry Kaiser (2017). Six records between 2006 and 2017. Norway recurs elsewhere in the catalogue too: Tonebod (2008) with Bugge Wesseltoft, Knut Buen and Anne Gravir Klykken; Bibi (2023) with composer Dai Fujikura on the Oslo label Jazzland; and Free Rangers, made with the Norwegian group Chrome Hill and released on Clean Feed on 20 March 2026.

Since 2014 her main vehicle has been Dōjō, a duo with drummer Tamaya Honda. Dōjō Vol. 1 (2014) brought in Nilssen-Love and trumpeter Nils Petter Molvær as guests; Dōjō Vol. 2 (2017) brought in Akira Sakata on alto saxophone and Keisuke Ohta on violin. The duo has since worked as a unit inside larger collaborations, with Gordon Grdina's The Twain (2022) and with Chrome Hill on two records. Her most expansive record under her own name alone remains Into The Forest (2019), where she plays 21-string koto, 17-string koto, electronics and voice, writes and arranges everything, and puts two drummers behind her, Tamaya Honda and Noritaka Tanaka, with Takashi Sugawa on bass and cello.

She is alive, working, and still touring internationally, with festival appearances including Moers in Germany in 2024.

Yagi's training is orthodox and her application of it is not. She plays the 13, 17, 18 and 21-string kotos and the shamisen, amplifies them, and runs them through electronics, so that the instrument can hold its own next to a drum kit or a saxophone at full volume rather than being placed decoratively on top of a Western ensemble. She treats the different string counts as genuinely different instruments with different jobs: the 17-string bass koto functions as a bottom voice, the electric 21-string as a lead one.

The formative influences are split across two traditions that rarely meet. On one side, Satomi Kurauchi and then Tadao and Kazue Sawai, the lineage responsible for dragging the koto into contemporary composition in the first place. On the other, the American experimental tradition she met in New York and at Wesleyan: John Cage, Christian Wolff, and John Zorn, who went on to produce her first solo record. Reviewers reach for the free jazz vocabulary when describing her playing with Brötzmann and Nilssen-Love, and for a more textural, composed vocabulary on records like Bibi and Into The Forest. Both are accurate, and the refusal to settle between them is the point. Her own summary of the ambition is "a genre unto myself."