Mabumi Yamaguchi (山口真文) was born on 1 September 1946 in Karatsu, in Saga Prefecture, and started on trombone in his junior high school brass band. At Keio University he took up the saxophone, the instrument he had long admired, and played for four years in the KMP New Sound Orchestra, the university's jazz big-band club. After graduating he spent about six months at the Yamaha Music Foundation before deciding to make music his work.

He began playing in Tokyo in 1969, at Ochanomizu Naru, which had just been opened by Katsuo Narita, and at the Shibuya club Oscar. In 1970, by his own account, he made his serious debut at Pit Inn and Taro in the quintet of the guitarist Ryo Kawasaki, still on alto. The following year the drummer George Otsuka recruited him and he switched to tenor. In the Otsuka quintet, with the trumpeter Shunzo Ohno, the pianist Toshiyuki Daitoku and the bassist Mitsuaki Furuno, he toured Japan widely.

His earliest recorded credit is from that year: on 25 September 1971 he played alto saxophone in the Brass 12 ensemble on Yosuke Yamashita's trio date Introducing Takeo Moriyama, a showcase for the drummer Takeo Moriyama. With Otsuka he then made two Three Blind Mice records on tenor and soprano: Go On, cut at Aoi Studio on 28 and 29 November 1972, and the double album In Concert!, recorded live at Kosei Nenkin Hall in Tokyo on 11 October 1973. He is also the tenor soloist on the second side of Now's the Time, recorded on 26 March 1974 at Toshi Center Hall, a shared date for Isao Suzuki, Sunao Wada, the Tsuyoshi Yamamoto trio and the Otsuka quintet.

In 1974 he left Otsuka and formed his own quartet, which worked mainly in live houses with the pianist Ichiro Doi, the bassists Kosuke Sakai and Ikuo Sakurai and the drummer Hideo Sekine. After the Rain (Union, 1976) was his first album as a leader, a quartet and quintet record with Kazumi Watanabe on guitar, Fumio Karashima on piano and electric piano, Nobuyoshi Ino on bass and Otsuka on drums. Earlier in 1976, on 7 February at the Nemuro Shimin Kaikan, he was one of two tenor saxophonists, with Yasuaki Shimizu, on the drummer Motohiko Hino's live quartet record Ryuhyo (Sailing Ice) for Three Blind Mice. His second album, Leeward (Union, 1978), was recorded in January 1978 with a new quartet of Doi, Sakurai and Sekine, and earned him the Shinjuku Jazz Award that year.

From 1976 he was also in the new group of the pianist Hiromasa Suzuki, first the Colgen Band and then The Players. He is the tenor saxophonist on Suzuki's Skip Step Colgen (Toshiba, 1977), with the bassist Akira Okazawa and the guitarist Tsunehide Matsuki, and on the CBS/Sony records Galaxy (1979), Wonderful Guys (1980, with Terumasa Hino on trumpet) and Madagascar Lady (1981). In the same busy years he played on the vocalist Kimiko Kasai's Love Celebration and Tokyo Special (both 1977), the singer Mieko Hirota's In My Feeling (1977), Ryuichi Sakamoto's Summer Nerves (1979) and the Chocolate album of 1982.

In November 1978 he joined an international band assembled around Otsuka. The Czech bassist Miroslav Vitous, the guitarist John Scofield and the young pianist Kenny Kirkland were in Japan, and over three days, 9 to 11 November, at Onkio Haus in Tokyo, they recorded Guardian Angels for Trio Records, with Yamaguchi on soprano saxophone. In the same weeks he recorded the Karashima album Hot Islands with Vitous and Otsuka. His own site describes touring Japan with that band in 1978 as a culture shock.

In 1981 the connection took him to New York. On 27 and 28 March at Sound Ideas Studios he recorded Mabumi (Trio Records) with Vitous, Kirkland and Tony Williams on drums, with Otsuka producing. The record lists him on tenor and sopranino saxophone. Soon afterward, by his own account, he left The Players and rejoined Otsuka's group Maracaibo, playing and composing with it until 1986. Its debut album, Maracaibo (Trio, 1980), credits him on tenor saxophone, with Vitous, Matsuki and the percussionist Nana Vasconcelos.

Squad followed in 1987: he, the pianist Toshiyuki Daitoku, the bassist Yoshio Suzuki and Hino recorded it at Sound Inn in Tokyo on 4, 5 and 6 February and released it on 21 July. Four of the eight pieces on the original LP are his. After that he worked as a sideman and bandleader in many settings: on Shigeharu Mukai's live album Mukai on the Wing (recorded January 1991), alongside the drummer Hiroshi Murakami; on the Shigeharu Mukai J Quintet featuring Junko Onishi record, cut in November 1993 with Junko Onishi, the bassist Rodney Whitaker and the drummer Gregory Hutchinson; and on the Lupin III albums of Yuji Ohno from 2004 and 2005. At the Moers festival in Germany on 14 May 2005 he played soprano saxophone in Masahiko Satoh's large ensemble, recorded as Live at Moers (Tribute to Masahiko Togashi), and his own site lists a 2009 appearance with Satoh's group at the North Sea Jazz Festival.

His own records in these decades are fewer and more deliberate. Regalo (1996) was self-issued as MAB-001, with Hiroshi Irie on keyboards, Nobuyuki Mizukami on bass and Taro Koyama on drums. Invitation (Skip, 1998) is a set of standards with Naoki Nishi, Mizukami and Tamaya Honda. Evening (2011) is a quartet with the pianist Mayuko Katakura, and Let Your Mind Alone (Spice of Life, 2017) is an all-original program with guitar. Since 2022 he has made three records with the Days of Delight label: the saxophone-bass-drums Trinity (recorded 11 December 2021), the soprano-saxophone quartet Viento (2023), and, on 27 March 2025, Next Standards, a duo album with Katakura that his own site presents as his first duo record. He is also on The One (2016), an album the Shigeo Maruyama Suikyo-za group made with the vocalist Jon Hendricks.

He turned 80 in September 2026 and is still working. His own site lists club dates in Tokyo, Nagoya, Chigasaki and Yokohama through the end of October 2026, and he takes students, from beginners to professionals, in a lesson program under his own name.

Yamaguchi is a tenor player with a modal and post-bop vocabulary, formed in the Tokyo club scene of the late 1960s. Collector listings describe Leeward as an outstanding modal jazz outing, with a long slow opener, "Dawn," built on a flanged Fender Rhodes, a piano duet in "Dewdrop," and a jazz-funk samba, "Distant Thunder," that gives each player a solo chorus. Discogs files that record under Modal, Fusion and Spiritual Jazz. He often switches to soprano, and on Viento plays nothing else.

The 1970s put him in two worlds at once. With Otsuka, Hino and Karashima he was part of the acoustic and spiritual-jazz current documented by Three Blind Mice and Trio, and with Hiromasa Suzuki's Players he was part of the studio fusion boom, writing for and soloing over electric rhythm sections.

His later repertoire looks back. Trinity (2022) is a pianoless trio on eight standards, from "Stella by Starlight" and "'Round Midnight" to "Solar." Next Standards (2025) takes on a second canon, the post-bop repertoire of the 1960s, among it "Nefertiti," "Footprints," "Dolphin Dance" and "Cantaloupe Island." Viento returns to his own writing: "Thalia" from the 1981 New York record, "True Face" and "Empty Mirror" from Regalo, and "Evening" and "Let Your Mind Alone" from his later records.