Masao Yagi (八木正生) was born in Tokyo on November 14, 1932, and went through Aoyama Gakuin High School. His first stages were the ones postwar Tokyo offered a young pianist who wanted to play American jazz: Club Rio in Tsukiji, and the United States military facilities around the capital, among them Tachikawa Air Base. English-language references add that he joined the Cozy Quartet in 1956, replacing Toshiko Akiyoshi when she left for the United States, and played there alongside Sadao Watanabe.

His earliest recorded credit is the 1957 King Records anthology Jazz Message From Tokyo (LKB-4). Yagi plays piano on two tracks, both with Watanabe on alto saxophone and flute: he arranged "Thou Swell" and wrote "Wherever I Go." The next year he was the pianist on two small-group records. Jazz Progression (Universal, November 1958) is a quartet date led by tenor saxophonist Hidehiko Matsumoto, and On Stage! Big 4 Plus 1 was recorded at Tokyo Video Hall on June 15, 1958, under the drummer George Kawaguchi, with Watanabe, Matsumoto and bassist Shinya Kimura. In 1959 he was recorded in a live broadcast session in Tokyo with the American clarinettist and baritone saxophonist Tony Scott, bassist Masanaga Harada and the 18 or 19 year old drummer Masahiko Togashi; the tape stayed unissued until 2015.

The record his name is attached to came in 1960. On July 13 and 14 of that year, at Bunkyo Public Hall in Tokyo, Yagi recorded Masao Yagi Plays Thelonious Monk for King Records (SKC 1), his first album as a leader. The quintet was Yagi, Sadao Watanabe on alto saxophone, trumpeter Akira Nakano, Harada on bass and Sadakazu Tabata on drums, and all eight tunes were written by Thelonious Monk: "Rhythm-A-Ning," "Off Minor," "'Round Midnight," "Misterioso," "Straight, No Chaser," "Blue Monk," "Monk's Mood" and "Evidence." Robin D. G. Kelley's Monk biography calls Yagi "Monk's greatest champion in Japan" in the years before Monk's first Japanese tour in 1963, and notes that the pianist had put several Monk tunes into the repertoire of the group he formed in 1959. Reissue-era sales copy describes it as one of the earliest Monk tribute albums recorded outside the United States.

The mid 1960s kept him close to the jazz scene. Jazz Inter-session (King, SKJ-1058) was recorded in Tokyo on February 8, 10 and 11, 1964, with the American alto saxophonist Charlie Mariano, Matsumoto, trumpeter Akira Fukuhara, Harada and the drummer Hideo Shiraki. His own group's Modern Jazz Blue Mood (Columbia, JPS-5048, August 1965) was a set of twelve standards played by a sextet with Akira Miyazawa on tenor saxophone and flute, trumpeter Norihiko Kitazato, guitarist Toru Konishi, bassist Masaoki Terakawa and George Otsuka on drums. He also arranged three tracks for the American singer Helen Merrill on Helen Merrill Sings Folk (London, 1966), contributed the arrangement of "Love For Sale" and played piano on the 1967 Swing Journal All Stars '67 album for Takt, and wrote four of the arrangements on Operation Sam Taylor (King, September 1967), a double-tenor record by Matsumoto and Miyazawa.

By then most of his work was for the screen. From the 1960s he scored films for the Toei studio, among them Yukinojo Henge (1963), many entries in the Abashiri Bangaichi prison-drama series between 1965 and 1971, the Furyo Banchou delinquent-gang series (1968 to 1973) and the Gokudo and Yakuza Keiji series. He composed and arranged the theme of the television anime Ashita no Joe in 1970, with lyrics by Shuji Terayama and vocals by Isao Bito, and the 1970 single of that theme credits him as composer. He arranged for the singer and actor Sonny Chiba, and for the vocal group the Duke Aces, whose 1975 album Yakeato no Jazz Vol. 1 (Toshiba) carries eight Yagi arrangements. For the 1976 Sonny Chiba film Dasso Yugi he wrote a score played entirely in jazz, the idiom Japanese sources describe him as regarding as his base. From 1980 to 1985 he contributed arrangements to the pop band Southern All Stars, and his name also appears on albums by singers including Mariko Takahashi and Yoshiko Goto. The Nescafé Gold Blend television commercial theme "Megame," with scat vocals by Kayo Ishu, was his as well.

Two late records bring the jazz side back into view. Inga (King Records, SKA-3021) was recorded on October 25 and 26, 1978, and released in 1979. Yagi composed and arranged all of it and plays piano and keyboards, with Miyazawa on tenor saxophone, Kiyoshi Sugimoto on guitar, Susumu Kazuhara on trumpet, Shigeo Suzuki on alto saxophone, Kunimitsu Inaba on wood bass, Michio Nagaoka on electric bass, Hideo Yamaki on drums and Osamu Nakajima on Latin percussion. It opens and closes with piano solos ("Memories" and "Sad Song") and includes a sextet reading of "Memories." In February 1985 he wrote the string arrangements for Miyazawa's ballad album 'Round Midnight - Amazing Jazz Ballad (Paddle Wheel), recorded at King Studio on February 6 and 7 with Masahiko Ozu on drums.

Yagi passed away on March 4, 1991, at the age of 58. His jazz recordings have turned up on reissue-era compilations of Japanese jazz, including Wax Poetics Japan's 2011 King of JP Jazz volumes, the 2019 collection J Jazz: Deep Modern Jazz From Japan 1969-1983 (Volume 2) and the King Records volume of Japanese Jazz Spectacle (2022).

Yagi's jazz identity was formed by bebop, and by Thelonious Monk in particular. The 1960 album is a working quintet with an alto saxophone and trumpet front line playing eight Monk compositions, among them an eight-minute "Straight, No Chaser" and a seven-minute "Misterioso," rather than a solo piano or trio tribute. His own group's 1965 Modern Jazz Blue Mood is a post-bop standards session built around Miyazawa's tenor saxophone and flute, in which "Lullaby of Birdland," "Autumn in New York" and "Night and Day" sit alongside "My Funny Valentine."

By 1979 the sound had moved. Inga places his writing over an electric bass and Latin percussion rhythm section, with Sugimoto's guitar and Miyazawa's saxophone as the principal voices, and Light in the Attic's catalogue copy describes it as modern Japanese jazz with a crossover sound shaped by his arrangements and melodies. The two solo piano tracks show the quieter, more lyrical side of his playing.

The larger part of his output sits outside that frame. Of the 195 releases Discogs lists under his name, only a handful are jazz leader dates; most of the rest are compilations, singles and albums of film, television, anime and pop music, and the credits that carry his name most widely are screen works such as Ashita no Joe. That is a real competing identity, but the sources that describe him treat jazz as the foundation he came from and returned to, from his Club Rio and Tachikawa years through the all-jazz Dasso Yugi score of 1976 to Inga.