Motoaki Masuo (増尾元章) was born in April 1951 into a musical family. His father was a bandleader and pianist, an elder sister is a pianist, and his elder brother is the guitarist Yoshiaki Masuo (born 1946), who went on to play in Sonny Rollins's band in New York. The family link is stated in Motoaki's own biography, which lists the pianist father and the guitarist elder brother, and in Japanese Wikipedia's article on Yoshiaki, which names Motoaki as his younger brother; the shared credits on Yoshiaki's records confirm that the two worked together. He took up the guitar in junior high school after hearing the Ventures, and he names the Beatles, Eric Clapton and the Rolling Stones as the influences that built his foundation as a musician. That is a rock and pop education rather than a jazz one, and the first half of his career followed it.

Around 1970, while studying composition at the Yamaha foundation's school in Ebisu, Tokyo, he served as the first instructor of Yamaha's rock-guitar class. With classmates from the composition course he formed a folk-rock group, Peace City, and made his professional debut. Five songs were recorded and released at the time, among them his own "Bara no Hana no Anata ni" and "Shitsuren no Uta"; both have since come back on CD, the first on the 2002 compilation Kissaten Rock: Expo and Soft Rock. In September 1973 Polydor issued his first solo album, First (MR 5035), on which he wrote the words and music, arranged, sang, and played electric guitar, electric bass and keyboards, with the songwriters Tetsuji Hayashi (林哲司) and Ken Sato (佐藤健), the drummer Kazuaki Misago (見砂和照) and, on harmonica, Masami Tsuchiya (土屋昌巳). The four musicians were also the pop and rock band Orange, which did not last, and he left it. The album is soft rock with folk touches, sung in Japanese, and it was reissued on CD in 2007.

By his account, he spent the mid-1970s composing and doing session work. He supplied songs to the singer Junko Ohashi: "Lucky Lady" and "Ai ni Sayonara" on her album Rainbow (Philips, 1977), on which he also plays piano and electric guitar, then "Rakujitsu Fukei" on Crystal City (1977), "Rush Hour Blues" on Hot Life (1980) and "Bellissima" on Postcard Fantasy (1982). Around 1977 and 1978 he was the first guitarist of the punk band S-KEN, led by Tadashi Tanaka, and Discogs credits him on guitar on two tracks of the 1979 CBS/Sony new-wave and punk compilation Tokyo Rockers. His biography adds that he played guitar behind Midori Kinouchi in concert, backed Katsumi Kahashi, Takao Kisugi, Akira Fuse and Hachidai Nakamura, and served as music director of the theatre troupe Tokyo Kid Brothers. The Hawaiian group Kalapana recorded his "Winds of Oahu", and he composed for Hong Kong singers (Leslie Cheung and Lu Fang appear in the credits in 1986) and for the Japanese idol Yukari Usami in 1984.

The turn toward instrumental music came through his brother. After leaving S-KEN in 1979 he travelled to New York for Yoshiaki Masuo's Good Morning (Electric Bird, recorded at Electric Lady Studios in September 1979), where he plays guitar and synthesizer, co-produced and co-arranged, and wrote two of the seven tracks, "Because of You" and "Inside Love". He joined Yoshiaki for the Japan dates that followed, and both brothers are on the electric-guitar front line of Masuo Live (Electric Bird, recorded at Kouseinenkin Hall, Tokyo, on 9 February 1980); he also plays on "Good Morning" on Masuo Special (1980).

In 1981 he formed his own band, Being, and began playing live. In 1982 King Records issued Natural Mind (K28A-238), which his own biography calls his first guitar-instrumental album. It is built around two playing techniques he developed himself, double bend and natural delay, and it carries the piece "Wind From Oahu". Riverpool Dreaming (K28A-284) followed later in 1982 and holds the soundtrack he wrote for the film Ningyo-girai (人形嫌い), starring the actress and singer Junko Mihara; in September 1982 Mihara released his song "Kutsuoto (Still Riverpool)" as a single. With A Smile (K28A-396) came out on 5 June 1983, and Happiness (K28P-6292), recorded at King Studio and his own studio between January and May 1984, brought in two rock guitarists, Takayuki Inoue and Kazuo Takeda, for "Shuffle Guitars". He wrote, arranged and produced all four King albums, and played nearly everything on With A Smile: electric, acoustic and slide guitars, ukulele, dobro, electric bass, piano, Hammond organ, synthesizer, marimba and whistle.

In 1985 the nerves in four fingers of his right hand were torn, and he retired from the guitar. In the years of recovery he produced events, composed and taught music. He returned to the stage in the early 1990s but withdrew again quickly, and in 2005 he came back as a guitarist for a second time; the 2008 version of his biography says he was working on his first solo album in more than twenty years. In July 2009 King issued the two-disc Motoaki Masuo Selection (NKCD-6472/3), 27 tracks drawn from the four King albums plus two of his compositions from Good Morning, some with light remixing and overdubs. "The Wind From Oahu" was later chosen for King's 2019 compilation No Matter What Fusion! The Best Fusion of King Records Treasures. His website announced a concert at Organ Jazz Club in Nakano, Tokyo, on 24 April 2020, which was cancelled. He turned 75 in April 2026.

Motoaki Masuo's place in Japanese jazz is a borderline one, and it is better stated plainly. His debut album is soft rock, his early work is folk-rock, punk and pop, and much of his output is songwriting and session guitar for pop and idol records. His jazz-related catalogue is the four King albums, which Discogs classifies as jazz, fusion and smooth jazz, together with his work on his brother's fusion albums.

Masuo is a rock-trained guitarist who arrived at fusion by way of pop songwriting. His influences are the Ventures, the Beatles, Eric Clapton and the Rolling Stones, not the jazz guitarists who shaped his brother, and the melodic, hook-led writing on the King albums shows it: mostly pieces of four to seven minutes, built around a lead guitar line and a bright studio sound, with synthesizers, acoustic piano, Latin percussion and, on Riverpool Dreaming, violin and harp. Discogs tags the King albums as fusion, smooth jazz, jazz-funk and Latin jazz.

His own account stresses technique. Natural Mind showcases the double-bend and natural-delay techniques he devised, and on With A Smile and Happiness he overdubs almost every part himself in his own studio, as composer, arranger, producer and multi-instrumentalist. There is little improvisation in the jazz sense; the records are composed and arranged guitar pieces with a pop sensibility. Before the guitar-instrumental albums, his own voice was that of a folk-rock and soft-rock singer-songwriter, which is what First documents.