In 2002, at the age of 13, Michishige participated in the Love Audition 2002 held by Up-Front Agency in search of new members for the Japanese female idol group Morning Musume. Tens of thousand of contestants auditioned, but only four where chosen in the final stage. Michishige, along with Eri Kamei, Miki Fujimoto, and Reina Tanaka were admitted to Morning Musume as sixth-generation members on January 19, 2003.

On July 16, 2003, the sixth-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”. 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 fourth-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, titled “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 titled Angels (エンジェルズ). 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 seventh-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 seventh 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 titled 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 consisted 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.

Michishige’s second solo photobook, titled Longing (憧憬, Doukei), was published on January 15, 2007. Shot in Okinawa, the book featured school uniform, swimsuit, simple one-piece, and wet yukata/bathrobe shots, in an attempt to convey a more mature image of the seventeen-year-old idol.

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 published in December 2007.

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 former 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).

When Risa Niigaki left on May 18, 2012, Sayumi Michishige was declared the new Morning Musume leader.

On April 29, 2014, during the Morning Musume ’14 Concert Tour Haru: Evolution concert in Yamaguchi, Michishige announced her graduation from Morning Musume and Hello! Project. She is set to leave during Morning Musume’s Fall 2014 concert tour. With her exit announced for Morning Musume’s Fall 2014 concert tour, the group consisted only of the three most recent generations since the introduction of the fourth-generation members in 2000. On November 26, 2014, she graduated from Morning Musume and Hello! Project, and in her place, Mizuki Fukumura was named group leader.