Tomita was born in Tokyo and spent his early childhood with his father in China. After returning to Japan, he took private lessons in orchestration and composition while an art history student at Keio University, Tokyo. He graduated in 1955 and became a full-time composer for television, film and theatre. He composed the theme music for the Japanese Olympic gymnastics team for the 1956 Summer Olympics in Melbourne, Australia.
In 1965, Tomita composed the theme song and incidental music for Osamu Tezuka's television animated series Jangaru Taitei (Jungle Emperor), released in the United States as Kimba the White Lion (with a different theme by Bernie Baum, Bill Giant and Florence Kaye). In 1966 he wrote a tone poem based on this music with an original video animation synchronized to the tone poem released in 1991. With Kunio Miyauchi, he created the music for the tokusatsu science fiction/espionage/action television series Mighty Jack, which aired in 1968. The same year, he co-founded Group TAC.
In the late 1960s, Tomita turned to electronic music with the impetus of Wendy Carlos and Robert Moog's work with synthesizers. He acquired a Moog III synthesizer and began building his home studio. He eventually realized that synthesizers could be used to create entirely new sounds in addition to mimicking other instruments. His first electronic album was Electric Samurai: Switched on Rock, released in Japan in 1972 and in the United States in 1974. The album featured electronic renditions of contemporary rock and pop songs, while utilizing speech synthesis in place of a human voice.
Tomita then started arranging Claude Debussy's impressionist pieces for synthesizer and, in 1974, released the album Snowflakes are Dancing; it became a worldwide success and was responsible for popularizing several aspects of synthesizer programming. The album's contents included ambience, realistic string simulations, an early attempt to synthesize the sound of a symphony orchestra, whistles, and abstract bell-like sounds, as well as a number of processing effects including reverberation, phase shifting, flanging, and ring modulation. Quadrophonic versions of the album provided a spatial audio effect using four speakers. A particularly significant achievement was its polyphonic sound, created prior to the era of polyphonic synthesizers. Tomita created the album's polyphony as Carlos had done before him, with the use of multitrack recording, recording each voice of a piece one at a time, on a separate tape track, and then mixing the result to stereo or quad. It took 14 months to produce the album.
In his early albums, Tomita also made effective use of analog music sequencers, which he used for repeated pitch, filter or effects changes. Tomita's modular human whistle sounds would also be copied in the presets of later electronic instruments. His version of "Arabesque No. 1" was later used as the theme to the astronomy television series Jack Horkheimer: Star Gazer (originally titled Star Hustler) seen on most PBS stations; in Japan, parts of his version of "Rêverie" were used for the opening and closing of Fuji TV's transmissions; in Spain, "Arabesque No. 1" was also used for the intro and the outro for the children TV program Planeta Imaginario (imaginary planet).
Following the success of Snowflakes Are Dancing, Tomita released a number of "classically" themed albums, including arrangements of: Igor Stravinsky's The Firebird, Modest Mussorgsky's Pictures at an Exhibition, and Gustav Holst's The Planets. Holst: The Planets introduced a science fiction "space theme". This album sparked controversy on its release, as Imogen Holst, daughter of Gustav Holst, refused permission for her father's work to be interpreted in this way. The album was withdrawn, and is, consequently, rare in its original vinyl form.
While working on his classical synthesizer albums, Tomita also composed numerous scores for Japanese television and films, including the Zatoichi television series, two Zatoichi feature films, the Oshi Samurai (Mute Samurai) television series and the Toho science fiction disaster film, Catastrophe 1999, The Prophesies of Nostradamus (U.S. title: Last Days of Planet Earth) in 1974. The latter blends synthesizer performances with pop-rock and orchestral instruments. It and a few other partial and complete scores of the period have been released on LP and later CD over the years in Japan. While not bootlegs, at least some of these releases were issued by film and television production companies without Tomita's artistic approval.
In 1984, Tomita released Canon of the Three Stars, which featured classical pieces renamed for astronomical objects. For example, the title piece is his version of Pachelbel's Canon in D Major. He credits himself with "The Plasma Symphony Orchestra", which was a computer synthesizer process using the wave forms of electromagnetic emanations from various stars and constellations for the sonic textures of this album.
Tomita performed a number of outdoor "Sound Cloud" concerts, with speakers surrounding the audience in a "cloud of sound". He gave a big concert in 1984 at the annual contemporary music Ars Electronica festival in Linz, Austria called Mind of the Universe, mixing tracks live in a glass pyramid suspended over an audience of 80,000 people. He performed another concert in New York two years later to celebrate the Statue of Liberty centennial (Back to the Earth) as well as one in Sydney in 1988 for Australia's bicentennial. The Australian performance was part of a A$7 million gift from Japan to New South Wales, which included the largest fireworks display up to that time: six fixed sound and lighting systems—one of those on a moored barge in the centre of a bay, the other flown in by Chinook helicopter—for the relevant parts of the show. A fleet of barges with Japanese cultural performances, including kabuki fire drumming, passed by at various times. His last Sound Cloud event was in Nagoya, Japan in 1997, featuring guest performances by The Manhattan Transfer, Ray Charles, Dionne Warwick, and Rick Wakeman.
In the late 1990s, he composed a symphonic fantasy for orchestra and synthesizer titled The Tale of Genji, inspired by the eponymous old Japanese story. It was performed by symphony orchestras in Tokyo, Los Angeles, and London. A live concert CD version was released in 1999 followed by a studio version in 2000.
In 2001, Tomita collaborated with Walt Disney Company to compose the background atmosphere music for the AquaSphere entrance at the Tokyo DisneySea theme park outside Tokyo. Tomita followed this with a synthesizer score featuring acoustic soloists for the 2002 film The Twilight Samurai (たそがれ清兵衛, Tasogare Seibei), which won the 2003 Japanese Academy Award for Outstanding Achievement in Music.
The advent of the DVD-Audio format allowed Tomita to further pursue his interests in multichannel audio with reworked releases of The Tale of Genji Symphonic Fantasy and The Tomita Planets 2003. He also performed a version of Claude Debussy's Clair de Lune for the soundtrack of Ocean's 13 in 2007.
In 2015, a number of tracks from Snowflakes are Dancing were featured on the soundtrack to Heaven Knows What, an American film directed by the Safdie Brothers. The same year, in recognition of his long career and global influence on electronic music, Tomita won the Japan Foundation Award, an award launched "to honor individuals or organizations who have made a significant contribution to promoting understanding and friendship between Japan and the rest of the world through academic, artistic and other cultural pursuits".
After suffering from heart disease for many years, Tomita died of heart failure in Tokyo on 5 May 2016.