She began listening to enka singer Eiko Segawa and studied classical Japanese dance, but as she loved dressing-up and make-up, she hoped to work in fashion magazines since she was too short to become a model.

Then, as a commemoration of one year since her debut, Nakashima released a Premium EP: Resistance, which hit the charts' number 1 spot for two weeks in a row.

In 2005, Nakashima co-starred alongside Aoi Miyazaki in the live-action movie Nana, based on the manga of the same name, which was released September 10, 2005.

She also sang one of the theme songs for the movie, which was released as a single under the name "Nana starring Mika Nakashima".

The single, titled "Glamorous Sky," was written by the manga's author, Ai Yazawa, and composed by L'Arc-en-Ciel vocalist Hyde.

On December 18, 2006, Nakashima and Yui Ichikawa came to New York City for the International Premiere of Nana 2 at the IFC Center.

Following the release of the Nana movie, Nakashima visited Memphis, Tennessee, and started another project beginning with the heavily gospel-influenced "Cry No More", the ending theme of the anime Blood+.

They released the single All Hands Together and as a result of her efforts, she was rewarded with an honorary citizenship of Memphis, Tennessee and a key to the city.

Nakashima's 23rd single "Life" as well as "Film Lotus VI", her sixth compilation of music videos, were released in August 2007.

"Life" was the theme song for the drama adaptation of the shoujo manga series of the same name by Keiko Suenobu.

Promotional videos were made for both Eien no Uta and its B-side, a cover of Cole Porter's "You'd Be So Nice to Come Home To".

A collaboration with owarai group Morisanchū (森三中) it was released under the name MICA 3 CHU (Mikasanchuu) and Nakashima claimed it was her way to "vent my complaints and enjoy doing something really silly.

[7] Nakashima's 31st single, "Always", was released on January 20, 2010, and was the theme song for the South Korean movie Goodbye, Someday (サヨナライツカ).

[9] Nakashima's 32nd single, "Ichiban Kirei na Watashi wo" (The Most Beautiful Me; 一番綺麗な私を) appeared on August 25, 2010, as the tie-in song to the TBS drama series, Unubore Deka (Conceited Detective, うぬぼれ刑事).

On October 22, 2010, she announced she would be pulling out of her first performance at the Nippon Budokan commemorating her 10-year career and that she would be taking a break from singing while recovering from having patulous Eustachian tubes in both her ears.

[11] In September 2012, Nakashima released the single Ashita Sekai ga Owarunara (明日世界が終わるなら, If the World Ends Tomorrow), which was the main theme for the Japanese version of the Resident Evil: Retribution film.

[12] On December 5, she released the single "Hatsukoi", which is the theme song for a live-action movie adaptation of the Kyō, Koi o Hajimemasu manga series by Kanan Minami.

[14] This same month she also collaborated with rapper Dohzi-T in a song titled "Dare yori mo", which was included on his album T's Music.

[15] On June 4, 2014, Nakashima released the collaboration single "Fighter" along with Miliyah Kato, which was chosen to be the theme song for the Japanese version of American film The Amazing Spider-Man 2.

After a hiatus of five years, she made a comeback to acting on September 11 in the TBS TV drama Omotesando High School Chorus Club.

On November 11, she started a new project group titled Mika Ranmaru and held a live event to unveil it.

It has also been covered by numerous artists including Joi Chua, Park Hyo Shin, Seo Young Eun, Han Xue, Vincy Chan, Eric Martin (Mr. Big's vocalist), Hayley Westenra, Hideaki Tokunaga and Gackt.