Mari Yaguchi became a member of Morning Musume during the second generation auditions, but resigned suddenly in April 2005. She basically cited her desire to move on "as an adult" with her career. The underlying reason that she had to quit was because it was discovered by the tabloid magazine FRIDAY that she was in a relationship with actor Shun Oguri though she was 22 at the time. Yaguchi and Oguri have reportedly broken up since April of 2006. Yaguchi's TV show Yaguchi Hitori (やぐちひとり ?) continues as before, and she now focuses on Japanese variety shows as guests and hosts rather than ventures on a singing career.
Early on in the group Yaguchi was often perceived as being "the short one" as she did not stand out a lot, but by the time 4th generation joined she had become one of the most active members of the group. Her most famous phrase is "Sexy Beam!" one of her solo lines from Morning Musume's "Koi no Dance Site" single, and she was asked jokingly in Joshi Kashimashi Monogatari, "When will your real sexy beam come out?". The "Sexy beam!" line has made its way around fans and fellow musicians alike. When she graduated from Morning Musume she was the last person to join before the year 2000 (Kaori Iida had left nearly 2 months before), and she currently has the fifth longest tenure of any member, with only Takahashi Ai, Niigaki Risa, Hitomi Yoshizawa and Kaori Iida being in the group longer.
Her first post-Morning Musume recording, "Nigiyaka na Fuyu", done as a collaboration with Berryz Kobo, appears as the coupling track on Berryz Kobo's November 2005 single "Gag 100 Kaibun Aishite Kudasai".
Yaguchi was the drama queen in a Japanese drama series called Sento no Musume (銭湯の娘!? ,daughter of public bathhouse?), Yaguchi was one of the main stars in a Japanese day time drama series called Gyarusa (ギャルサ ,Gal Circle?), along with her show Yaguchi Hitori, and MCing Hello! Project concerts. From May 25, 2007 and up to June 6, 2007, Yaguchi performed in "Damn Yankees", a popular 1955 Broadway musical at the Tokyo Aoyama Theater.
In summer of 2007, Yaguchi was recruited to host a new Gyao TV show, "Midtown TV", becoming her third television show on air. "Midtown TV" airs all weekdays and Yaguchi hosts it on Thursdays. Also, Mari and seven H!P Eggs are scheduled to appear live and with audience on "Ciao TV", a Kids Station show, on 09/02, 19h~19.30h, on what appears to be yet another TV show. With that commencing, Yaguchi will be in four TV shows being aired simultaneously.
It was announced October 19, 2008, on the official Hello! Project website, that Yaguchi, together with the entire Hello! Project Elder Club, will graduate March 31, 2009. A graduation concert was held at Yokohama Arena on February 1, 2009.
Following the popularity of singles and an album released in conjunction with the Japanese variety program, Quiz! Hexagon II (クイズ!ヘキサゴンII ?), Yaguchi released her debut single, "Seishun Boku/Seishun Ore" (青春 僕/青春 俺 ?), four years after her final release in Morning Musume. She performed the song for the first time on Quiz! Hexagon II on February 25, 2009