Haruka Shimotsuki started her career in 2001 beginning with singing main themes for games, and writing and composing songs. She started releasing her dōjin music under the names Maple Leaf and tieLeaf; tieLeaf is a collaborative circle with Ao Sorano and Nao Hiyama. The group has a seventy-two-page dōjinshi, one CD entitled Tsukioi no Toshi (also called Leozet Lag Ecliss in its own original language, the Lag-Quara language), a manga and novel text story book that was twenty-eight pages, and a single CD with the same name as the book, Lip-Aura, a side-story of Tsukioi no Toshi. Another project by tieLeaf, an adventure titled Neji Maku Tsuki, serves as a tie-in to Tsukioi no Toshi.
She gained attention after singing the ending theme "Tōmei Shelter" for Rozen Maiden under the name Refio+Shimotsuki Haruka. After singing "Tōmei Shelter", she and Myu (a member of Refio) formed the band Kukui. Shimotsuki has also formed a group with Rekka Katakiri and Chata many times, and they have been good friends for a long time. As an independent singer, she was once a supporting member of Sound Horizon, and made an appearance as a guest singer in their 2006 concert. She took a more major role in Sound Horizon's album Moira and performed in the corresponding live concerts in 2008 and 2009. Haruka Shimotsuki held a solo concert, Tokeidai no Aru Machi yori (時計台のある街より?): Haruka Shimotsuki Solo Live Lv.1 on August 10, 2006. Her second solo concert, Haruka Shimotsuki solo live Lv.2: Shimotsuki Haruka wa Level ga 1 Agatta (シモツキンはレベルが１あがった?), was held on July 13, 2008.
Besides singing, composing and writing the lyrics for the opening and ending themes for the PC visual novel Relict: Toki no Wasuremono, she also composed about seven background music tracks for the game. Wind and string instruments are heavily used for the tracks, and she was able to exhibit her talent in ethnic music. This was her first time in composing songs for a game, and her feelings at that time were recorded in the booklet attached to the Image Soundtrack of the game. In the PlayStation 2 game Ar tonelico: Melody of Elemia, she composed and performed the songs of one of the heroines, Aurica Nestmille, and took part in singing the ending theme "EXEC_PHANTASMAGORIA/.". She later took part in the sequel Ar tonelico II Sekai ni Hibiku Shōjo-tachi no Metafalica as the singing voice of one of the heroines, Luca Truelywaath, and took part in the ending song "EXEC_with.METHOD_METAFALICA/.".
In November 2009, she had another concert, Haruka Shimotsuki Original Fantasy Concert 2009 -FEL ARY ARIA-, was held at NIPPON SEINEN-KAN HALL featuring all her songs from Tindharia no Tane and a select few from Griotte no Nemurihime. It was later released on DVD on February 24, 2010.