Chieko Shiomi changed her name to Mieko for divinatory reasons between 1967 and 1969. In 1957 she began studying musicology at the Tokyo National University of Fine Arts and Music. Interested in avant-garde music, she founded the group Ongaku in 1960, which explored new forms and improvised performances. She then played her first compositions in public. In 1961 she participated in a concert with Toshi Ichiyanagi, the first husband of Yoko Ono, who mentioned the group to George Maciunas. He in turn included them in his tentative program for the Festival of Very Early Music, a programme that he wrote in 1962 but which was never realised; the group split up shortly after. Mieko Shiomi deepened her exploration of spatial-temporal music in relation to everyday objects. Between 1962 and 1963 she began to write exclusively textual pieces that she called action poems. In 1963 she created Endless Box, a multiple composed of 34 boxes in foldable paper of differ sizes, which she sent – on the advice of Nam June Paik – to Maciunas, thus making her entrance into the Fluxus movement.
Arriving in New York in 1964, she participated in the Fluxus Symphony Orchestra at Carnegie Hall. Over the next year, she performed several actions such as Disappearing Music for Face. Her works combined music, visual arts and poetry, as seen in her most celebrated piece Spatial Poem, a series of nine mail art actions (1965-1975) where she sent an idea for a simple action to several people and asked each of them to react with a performance and a text, and to send her the documentation. This work allowed her to carry out a simultaneous actions all over the world and to implement real human exchange. After returning to Tokyo in 1965, she spread Fluxus ideas in Japan and concentrated more and more on new technologies. From the 1990s, she reinterpreted Fluxus works, as with the Fluxus Media Opera (1994).