Yoshiaki Masuo (増尾好秋) was born on 12 October 1946 in Nakano, Tokyo, the son of a jazz bandleader and pianist, Hiroshi Masuo (増尾博). He has said he never had formal musical training. He picked up the guitar on his own at 15, and the players he names as early influences are Wes Montgomery and Grant Green; Japanese Wikipedia adds Barney Kessel as the one who turned him toward jazz. His younger brother is the guitarist Motoaki Masuo (増尾元章), who plays on three of his Electric Bird albums.

At Waseda University he played in the jazz club, and the alto saxophonist Sadao Watanabe took notice of him, according to Japanese Wikipedia at a Yamaha-sponsored contest. By his own account he joined Watanabe's group at the end of 1967, which put him on the leading Japanese jazz stage while he was still a student, and he stayed for about three years, touring outside Japan and playing the Montreux and Newport festivals. Discogs credits him on Watanabe's Pastoral (CBS/Sony, 1969) and on the live album At Montreux Jazz Festival (1971), and in 1968 he is already on guitar on trumpeter Terumasa Hino's Feelin' Good, arranged by Masabumi Kikuchi. His official biography says he topped the guitar category of the Swing Journal readers' poll in 1970 and won it five times in all.

His first record as a leader was Winds of Barcelona (CBS/Sony, 1969), recorded on 19 February and 1 May 1969 with Yoshio Suzuki on bass, Kiyoshi Sugimoto as second guitar, Hideo Miyata on flute, Kazuo Yashiro on organ, Nobuhiro Suzuki on vibraphone and marimba, Fumio Watanabe on drums and Larry Sunaga on percussion. A second album, 24, followed in October 1970 for CBS/Sony, with Suzuki again on bass, Motohiko Hino among the drummers, Hideo Ichikawa on electric piano, a brass section on two tracks, and Watanabe guesting on flute on one. In January and February 1970 he was also one of four guitarists on Guitar Workshop for Union Records, recorded at Teichiku Kaikan Studio alongside Sugimoto, Ryo Kawasaki and Masayuki Takayanagi.

He moved to New York on 13 June 1971. His official biography lists early associations there with the bassist Teruo Nakamura, Lenny White, Michael Brecker and Chick Corea, and a stretch as a regular with the songwriting duo Ashford and Simpson. The documented session of that year is Elvin Jones's Merry-Go-Round (Blue Note, 1972), on which he plays guitar on two tracks, "'Round Town" and "A Time For Love", in a band that includes Dave Liebman, Steve Grossman, Jan Hammer and, on "A Time For Love", Corea. His official biography also puts him in Lee Konitz's group in 1972.

In the spring of 1973 he joined Sonny Rollins, and the Rollins years are the part of his career that the rest of the jazz world knows. The first stint ran three years, with Bob Cranshaw on electric bass, David Lee on drums and Mtume on percussion. He is on Sonny Rollins in Japan (Victor), recorded on 30 September 1973 at Nakano Sun Plaza in Tokyo; on Horn Culture (Milestone, 1973); and on The Cutting Edge (Milestone, 1974), recorded in concert at the 1974 Montreux Jazz Festival with Stanley Cowell on piano. He returned to the band in the early 1980s: Rollins's Reel Life (Milestone, recorded 17 to 22 August 1982 at Fantasy Studios in Berkeley) has him on electric guitar for five tracks and acoustic guitar on a sixth, beside Bobby Broom, Cranshaw and Jack DeJohnette, and his biography says Rollins's band plays a Masuo composition on it, "Sonny Side Up". His site counts about seven years with Rollins in total. Between the two periods he toured Europe with Elvin Jones, played with the organist Larry Young and formed his own electric fusion group.

In New York he also made his own records. 111 Sullivan Street (East Wind, recorded 27 and 28 September 1975) is the first, an acoustic-leaning set of standards and originals with "God Bless the Child" and "Without a Song". The turning point came when the new Electric Bird label, an arm of King Records in Tokyo, signed him in 1977. Sailing Wonder (SKS-8001), recorded on 15 November 1977 and issued as the label's first release, brought in Steve Gadd on drums on three tracks, Eric Gale on electric guitar, Richard Tee on keyboards, T.M. Stevens on electric bass, Dave Grusin and Mike Nock on synthesizers, and a closing track titled "Viento Fresco (For Sonny)". It was a hit, and he made a run of fusion albums for the label: Sunshine Avenue and Good Morning (both 1979, recorded at Electric Lady Studios, the first with Horace Ott's string arrangements), Masuo Live (1980), The Song Is You And Me (1980, with Jan Hammer, Michael Brecker and Randy Brecker) and Finger Dancing (1981), a full record with Hammer, who also joined him for live dates. Mellow Focus (Casablanca, 1982), produced with Shirley Masuo, added Kenny Kirkland, Will Lee and Bob Cranshaw.

Around those records he stayed in demand as a sideman. He is the electric guitarist on Kazumi Watanabe's Village in Bubbles (1978), credited to Kazumi Watanabe with the New York band Manhattan Blaze, and on that group's Venus Eyes (1979), recorded at Sound Ideas Studios in October 1978 with Idris Muhammad, Joe Chambers and Eddie Henderson. In May 1978 he joined George Otsuka's New York session for Maracaibo Cornpone, and in June 1981 he recorded New York Concerto at Van Gelder Studio in New Jersey with the drummer and vibraphonist Joe Chambers, Sonny Fortune, Kenny Barron, Eddie Gomez and Ray Mantilla. He also appears on Takehiro Honda's It's Great Outside (1978, recorded in New York with Cornell Dupree) and on Kunihiko Sugano's Manhattan Focus (1979).

From the middle of the 1980s he owned a recording studio in SoHo, The Studio, and the studio nearly displaced the guitarist. He recorded and mixed a one-man album, Masuo (1989), on which he was composer, arranger, engineer and mixer, and he became the in-house producer for the Japanese labels Jazz City and Jazz City Spirit. Discogs lists him as producer on 49 releases, nearly all from 1988 onward, and co-producer on three more, among them Fred Hersch's Evanessence: A Tribute to Bill Evans (recorded August 1990), the Marc Cohen quartet record My Foolish Heart with Gary Peacock and John Abercrombie (1988), and dates by Steve Kuhn, Marc Copland, Kenny Drew Jr., Larry Willis and Mark Soskin. His own records of the period are fewer: the acoustic duo Lush Life - Billy - Tokyo with the bassist Tsutomu Okada (Pony Canyon, 1990), A Subtle One (Jazz City, 1991) with Ron Carter, Al Foster, Grady Tate, Kenny Kirkland and Romero Lubambo, with liner notes by Ira Gitler; Just Like Old Times (Jazz City Spirit, 1993, with Watanabe on alto saxophone, Yoshio Suzuki, Buddy Williams and Kenny Drew Jr.); and Are You Happy Now (Omagatoki, December 1998) with Larry Goldings on organ and Lenny White on drums.

He closed The Studio at the end of January 2008 and returned to playing full time. Life Is Good (August 2008) was the first release on his own Sunshine Ave. label, with Lenny White, Larry Goldings, Yoshio Suzuki and T.M. Stevens, and he sings on several tracks. I'm Glad There Is You (2009) is a guitar and piano duo with Bill Mays, and Around the World (2011) is a duo with Suzuki, his old Waseda schoolmate. A remixed edition of Life Is Good came out in October 2024.

In 2012 he formed a working band in Japan for the first time in more than three decades, Power Spot, and Japanese Wikipedia records a successor unit, MAGATAMA, from 2016, renamed the Yoshiaki Masuo Group (YMG) in 2023. His official site says the band has changed members and names several times and calls it the Yoshiaki Masuo Band in 2026. He is based mostly in the New York area and, in recent years, spends roughly three months in Japan every spring and autumn. He turned 80 on 12 October 2026, and the site shows photographs from a birthday concert at Shinjuku Pit Inn that day, in the middle of an autumn tour of Japan clubs.

Masuo names Wes Montgomery and Grant Green as his first influences, and Japanese Wikipedia adds Barney Kessel; all three come from the mainstream and hard bop tradition. His own career has run on two tracks. In the Electric Bird years he writes and arranges fusion for a New York rhythm section (T.M. Stevens on bass, Steve Gadd or Robbie Gonzalez on drums), with Fender Rhodes, strings and synthesizers around the guitar, and he adds acoustic guitar, keyboards, percussion and vocals to his own tracks. In the Jazz City years and since, the writing is closer to post-bop: A Subtle One is a session of standards and originals with Ron Carter and Al Foster, and I'm Glad There Is You is a guitar and piano duo.

The Rollins years are the other pole. On Horn Culture, The Cutting Edge and Reel Life he is the only guitarist (on Reel Life, one of two with Bobby Broom) in a small band led by a tenor saxophonist, and his own albums keep some of that small-group, open-ended feel. He wrote "Viento Fresco (For Sonny)" for Sailing Wonder and "Minor Samba (For Sadao San)" for Mellow Focus, two titles that name the bandleaders who shaped him. His 1989 album Masuo shows a third side: a one-man production in his own studio, with synthesizers and overdubbing, in which he was composer, arranger, engineer and mixer.