Tanioka was born in Hiroshima, Japan, where she studied music and composition while in school and enjoyed listening to video game music as her younger brother was a gamer. Among the composers she grew familiar with were Square employees Hitoshi Sakimoto, Nobuo Uematsu and Kenji Ito. Her favorite classical composers at the time were Piotr Paleczny and Hiromi Uehara. Tanioka attended Kobe University, where she studied music and joined a choir. She graduated with a degree in musical performance. Although she had planned on performing music as a career, during college she became more interested in composing than performance, and in video game composition because of her childhood experiences. After graduating, she joined Square (now Square Enix) as a composer in 1998.
Her first score was the soundtrack to 1998's The Fallen Angels, by developer Steel Hearts, which she composed with Masaki Izutani. That same year, she joined Square and composed her first soundtrack for them to a game in the Chocobo series, Chocobo's Dungeon 2, with Yasuhiro Kawakami, Tsuyoshi Sekito and Kenji Ito. Her second work in the series was also her first solo soundtrack, that of Dice de Chocobo, a video game adaption of a board game. She composed for two other projects over the next two years, All Star Pro-Wrestling with Tsuyoshi Sekito and Kenichiro Fukui and Blue Wing Blitz by herself. Her first major composing role was in 2002, when she was one of three composers chosen to write the soundtrack to Final Fantasy XI. Although she did not contribute more than one song to the multiple expansions to the game, during this period she joined The Star Onions, a band made of Square Enix composers that arranges and performs Final Fantasy XI music. The group has released two albums to date.
After Final Fantasy XI, Tanioka returned to the Chocobo series to arrange her Dice de Chocobo soundtrack for the game's remake, Chocobo Land: A Game of Dice. After that, however, she composed the soundtrack to Final Fantasy Crystal Chronicles, which led to her composing the soundtracks to the other five games in the series. Between the games, she worked on the soundtracks to Code Age Commanders: Tsugu Mono Tsuga Reru Mono, Code Age Brawls, Project Sylpheed, and Final Fantasy Fables: Chocobo's Dungeon. On February 28, 2010 Tanioka announced her departure from Square Enix, following several other Square Enix composers such as Kenichiro Fukui, Junya Nakano, and Masashi Hamauzu; she joined the composer's group GE-ON-DAN with many other composers like Nakano. In 2011, she became one of the founding members of Ringmasters, a non-exclusive worldwide group of artists and composers, though she later left the group. She composed in 2011 music for iOS interactive storybooks, Snow White, The Ugly Duckling, and Hansel and Gretel, before founding her own independent company, Riquismo, in August 2012. Riquismo is not a full studio, and Tanioka continues to work independently. Tanioka has said that she decided to become a freelance composer because she wanted to write for a wider variety of subjects. Since its founding she has composed music for Ragnarok Odyssey, for which she wrote independently of any prior music from the Ragnarok series, and MA.YU.MO.RI.
Unlike the soundtracks to the numbered Final Fantasy games, no songs from the Crystal Chronicles or Chocobo soundtracks have appeared in any compilation albums produced by Square Enix. Songs from the two series have also not appeared in any of the official Final Fantasy music concerts, although "Morning Sky" from the Crystal Chronicles soundtrack was played in the first Games in Concert performance in Utrecht, Netherlands on November 26, 2006. It was performed by Floor Jansen of the band After Forever and the Metropole Orchestra.
Kumi Tanioka's signature style, especially for the Crystal Chronicles games, is that of "world music". She has described the musical style for the soundtrack to Final Fantasy Crystal Chronicles as being based on "ancient instruments". The soundtrack has extensive use of many medieval and renaissance musical instruments like the recorder, the crumhorn and the lute, creating a distinctively rustic feel, and also follows the practices and styles of medieval music. She says the idea came to her while looking at illustrations of the game world, which gave her the idea of making "world music", where the tracks would "not limited to a single country or culture". She feels that specific instruments do not necessarily need to be tied to a specific geographic region, and tries to see, for example, how an Indian instrument and a Celtic instrument might work with each other.