He is often regarded as one of the most influential innovators in chiptune and video game music, producing music in a number of genres including rock, jazz, symphonic, and various electronic genres such as house, electro, techno, trance, and hip hop.

[1][2][3] Koshiro and his sister Ayano founded the game development company Ancient in 1990, of which he remains the president.

The sequencing skills and experience he gained from this would later be utilized in his early video game projects.

Falcom used compositions from the PC-8801 demo tape he had sent them in their Dragon Slayer action role-playing game Xanadu Scenario II, for its opening theme and several dungeon levels.

He then produced the soundtrack to Dragon Slayer IV / Legacy of the Wizard (1987), which was influenced by the sounds of early Konami games.

[1] His most notable freelance work was for Sega: his first freelance work for the company was the soundtrack to The Revenge of Shinobi (1989), for which he produced house[1][18] and "progressive, catchy, techno-style compositions"[7] that fused electronic dance music with traditional Japanese music.

[12] His sister Ayano has designed characters and graphics for several games Koshiro has worked on, including the Streets of Rage (Bare Knuckle in Japan) series, Ys, and ActRaiser.

[1] His soundtracks for the Streets of Rage series (known as Bare Knuckle in Japan) from 1991 to 1994 were composed using then outdated PC-8801 hardware alongside his own original audio programming language.

[21] The soundtrack for Streets of Rage 2 in particular is considered "revolutionary" and ahead of its time,[8][9] for its "amazing blend of swaggering house synths, dirty" electro-funk and "trancey electronic textures that would feel as comfortable in a nightclub as a video game.

They described Koshiro as "just about universally acknowledged as the most gifted composer currently working in the video game field.

"[23] Koshiro composed the soundtrack to Streets of Rage 3 (1994), along with colleague Kawashima who contributed in a larger capacity than in 2.

He created a new composition method called the "Automated Composing System" to produce "fast-beat techno like jungle.

[1] This resulted in innovative and experimental sounds generated automatically that, according to Koshiro, "you ordinarily never could imagine on your own."

This method was very rare at the time, but has since become popular among techno and trance music producers to get "unexpected and odd sounds.

[25] Also in 1994, Koshiro co-composed the soundtrack with Kawashima for the Mega-CD version of Eye of the Beholder, a dungeon crawl role-playing video game ported over from the original by Japanese developer Opera House and published by Sega.

For the Wangan Midnight series in particular, his compositions were mostly trance music, a style he was previously unfamiliar with.