Sayumi Michishige, 13 years old, officially became a 6th generation member of Morning Musume, along with Eri Kamei, Miki Fujimoto and Reina Tanaka, on January 19, 2003, when the results of the Love Audition 2002 were made public. Just a few weeks later, she was placed in Morning Musume Otomegumi subgroup, when the group was temporarily divided so it could perform in smaller towns, thereby reaching out to a greater number of fans who couldn't otherwise have the privilege of watching them perform live. During the spring she, along with the rest of the 6th generation members, made her first concert appearance on the Morning Musume Concert Tour 2003 Spring "Non Stop!" during Kei Yasuda's graduation.
On July 16, 2003, the 6th generation members (with the exception of Miki Fujimoto) released their first photobook and corresponding DVD, both shot in Hawaii, as part of the Alo Hello! (アロハロ!?) series. Their musical debut came shortly thereafter, on July 30, with Morning Musume's 19th single Shabondama. Although Michishige didn't get many solo lines in the song, one of those instances—in which she shouts "Shabondama"—did get her some attention on TV shows. Towards the end of 2003 Michishige began appearing in the show Revelations of M (Mの黙示録, M no Mokushiroku?), as a regular with ex-Morning Musume member Yuko Nakazawa.
2004 saw her first appearance on an album, Best! Morning Musume 2, and in a movie—"Star Sand Island, My Island - Island Dreamin" (星砂の島、私の島－～Island Dreamin'～, Hoshisuna no Shima, Watashi no Shima - Island Dreamin?). During the summer Michishige appeared in her first solo photobook and was placed in a temporary unit called Ecomoni with 4th generation Morning Musume member Rika Ishikawa. Originally, the goal of the group was to promote environmental awareness only for the duration of the "Cool Down the Hot Earth" (熱っちぃ地球を冷ますんだ, "Atchii Chikyū wo Samasunda"?) festival, but their popularity led to the members being cast as voice actors for the fourth Hamtaro movie—along with Hello! Project soloist Aya Matsuura, with whom they also released the main theme for the film, entitled Tensai! Let's Go Ayayamu (天才！Let's Go あややム, Genius! Let's Go Ayayamu?)—and appeared in subsequent festivals.
In 2005, Michshige and Rika Ishikawa released a duo photobook entitled Angels (エンジェルズ?). Though it has been called an Ecomoni photobook because both members appear in it, officially Angels has nothing to do with the unit. Michishige also appeared as a regular news anchor on Hello! Morning for most of the second half of 2005. That same year she was assigned to be a mentor to 7th generation member Koharu Kusumi, with whom she formed a duo called Rainbow Pink in the beginning of 2006. Introducing themselves as "Shige-pink" and "Koha-pink", the group was featured on Morning Musume's 7th album, Rainbow 7, with the song Rainbow Pink (レインボーピンク?), and subsequently on the 7.5 Fuyu Fuyu Morning Musume Mini! and Sexy 8 Beat albums, with Wa~Merry Pin Xmas! (わ～MerryピンXmas!?) and Takara no Hako (宝の箱, Treasure Box?), respectively.
Following in fellow Morning Musume member Miki Fujimoto's footsteps, Michishige began hosting a radio show entitled "Hypernight - Michishige Sayumi Konya mo Usa-chan Peace" (ハイパーナイト モーニング娘。道重さゆみの今夜も♥うさちゃんピース?) in October 2006. Running approximately 30 minutes per episode, Michishige's show consists of her answering fan mail and participating in several games, among which saying a given phrase/expression in reverse, and giving general advice to her listeners.
In early December 2006 it was announced Michishige would be given a second solo photobook, entitled Longing (憧憬, Doukei?), released shortly thereafter on January 15, 2007. Shot in Okinawa, the book featured school uniform, swimsuit, simple one-piece and even wet yukata/bathrobe shots, in an attempt to convey a more mature image of the seventeen year old idol.
Michishige has also starred in "Bow30°" (「おじぎ30度」?), an original internet drama created for the purpose of advertising the Family Restaurant chain Gusto (ガスト?), as Ruruka Chī (地井ルルカ?), a novice waitress training under the supervision of her seniors Karin Kinoshita (木下花梨?) (played by Morning Musume's Ai Takahashi) and Mika Mochizuki (望月深華?) (Reiko Tokita).
On June 9, 2007, Michishige began co-hosting the Young Town radio show as a result of Miki Fujimoto's Friday magazine scandal and subsequent resignation from Morning Musume. Shortly thereafter she released her third solo photobook and DVD, 17: Love Hello, both shot on the island of Guam. Her fourth solo photobook, Sōsō was released in December.
In 2008, like in previous singles (a notable exception being Namida ga Tomaranai Hōkago), Michishige was given a relatively minor role both in Resonant Blue and Pepper Keibu, with the exception of a solo line in the latter. Her radio show was extended from 30 to 60 minutes and she also released her fifth solo photobook.
On January 3, 2009, for the first time since she joined Morning Musume, Michishige appeared as an independent talent on a TV show, namely the Otona Gakuryoku Kentei SP (大人学力検定スペシャル, Adult Knowledge Test Special?) quiz show. A cast of 30 performers (including former v-u-den member Yui Okada and second generation Morning Musume member Mari Yaguchi) gathered to answer a series of questions taken from elementary school textbooks. From a total of 60 questions she managed to score 22 (37%) and placed 29th. Later that year, Michishige was assigned to be a part of the new shuffle group zoku v-u-den, a revival of v-u-den, along with Risako Sugaya (of Berryz Kobo) and Junjun (also of Morning Musume).
The phrase Usa-chan Peace (うさちゃんピース, Bunny Peace?) has become a trademark of Michishige. While saying this phrase, she places her hands next to her ears and holds her index and middle fingers in V shapes to imitate rabbit ears. According to Michishige, Usa-chan Peace became her trademark pose gradually. At first she used it when taking print club photos with her sister, but after seeing how cute it looked, she started adopting it on official photoshoots and related events.
The Usa-chan Peace pose has been widely reproduced in Hello! Project productions (such as the aforementioned Bow 30° drama, in the second episode of the first series) and events, but also outside the scope of Tsunku's all-girls franchise, most notably in the 132nd Ryūnosuke Akutagawa prize award ceremony. For the commemorative photograph of the event, Kazushige Abe (阿部 和重?) held his hands in a manner patterned after the Usa-chan Peace sign, in a clear homage to Michishige's invention.