It started with Revo releasing his music creations on the internet on his website in the late 1990s. In 2001, Sound Horizon participated Comic Market as part of a DojinMusic circle and released their first story CD, Chronicle, an all-instrumental track CD, with occasional narration, background chorus and sound effects. The inclusion of actual singing began from their second release onwards. Their subsequent works were released at Comic Market and M3.
Sound Horizon's first major release was in 2004, with the album Elysion ~Prelude to Paradise~. Their first maxi-single, "Shōnen wa Tsurugi wo...", which was released in 2006, includes "Shūtan no Ou to Isekai no Kishi ~The Endia & The Knights~", the theme for the Playstation2 simulation RPG Chaoswars, and "Kamigami ga Aishita Rakuen ~Belle Isle~", the opening theme for the MMORPG Belle'Isle.
On September 3, 2008, Sound Horizon released the album Moira.Moira debuted at No. 3 on the Oricon weekly album charts, selling over 45,000 copies in its first week.
Sound Horizon released the single "Ido e Itaru Mori e Itaru Ido" on June 16, 2010. The single featured guitarist MartyFriedman and Miku Hatsune along with a beta voicebank for Vocaloid known as "Junger März PROTOTYPE β".