During her third year in middle school, under her mother's influence, Yui began a journal of her everyday feelings and poetry. By the time she was in high school, she was writing songs. She decided that school and music could not coexist. She encountered a live street performance and expressed her desire to pursue a musical career to the band, Bianco Nero, at the end of the concert. Bianco Nero advised Yui to join a private music school.
Her professional career began in March 2004 when, at the recommendation of her juku instructors, she applied for an audition hosted by Sony Music Japan.Because "I Know" was incomplete at the time, the judges were able to get a glimpse of what would later be dubbed "Yui-go", or Yui-speak (Yui語 in Japanese), nonsensical English hummed to a tune during her songwriting process — an example of Yui-go can be found in the film Midnight Sun when Yui is working on the song "Goodbye Days". Despite the audition rule that a participant could only perform two songs, Yui was allowed to sing three. She first sang "Why Me" (a song later included in her debut single), followed by "It's Happy Line" and "I Know". The judges gave her the maximum score, causing a fierce scramble among record labels to sign her.
On December 24, 2004, she released her debut single "It's Happy Line" under indie label Leaflet Records, coupled with the track "I Know". The pressing was limited to only 2,000 copies in her home area.
From Me to You (2005–2006)
On February 23, Yui released her first major debut single, "Feel My Soul". The music in Fukigen na Gene was based on her songs such as "Feel My Soul" and "It's Happy Line". With the publicity the drama tie-in drew in, "Feel My Soul" managed to sell over 100,000 copies and managed to chart at number 8 on Oricon Weekly Charts in its first week. Her next three singles, "Tomorrow's Way" (theme song for the Japanese movie, "Hinokio"), "Life" (fifth ending song for a popular anime, Bleach), and "Tokyo" did not chart as high as Feel My Soul did and were only moderately successful in comparison.
Can't Buy My Love (2006–2007)
Yui made her acting debut in the full-length Japanese feature film Midnight Sun (Taiyou no uta, タイヨウのうた), which opened on June 17, 2006. The film was screened at the 2006 Cannes Film Festival. Yui attended the Japan Academy Prize and received the Best Newcomer Award for Midnight Sun. Prior to the release of the movie, Yui released her fifth single "Good-bye Days" written specifically for the film. The single is by far her highest selling at more than 200,000 copies sold. All three tracks on the single are featured in the "Midnight Sun." The single also re-released the track "It's Happy Line," originally released on her debut single.
Her next single, "I Remember You", did well in terms of sales, riding on the wave of the popularity of "Good-bye Days." Yui's seventh single "Rolling Star" was chosen as the 5th opening theme for the anime series Bleach. Her eighth single, "Cherry" was featured in commercial promoting KDDI au Listen Mobile Service.
She released her second album Can't Buy My Love on April 4, 2007. The album spending two weeks at number 1 on the Oricon charts, shattering her entire previous album's record sales in one week. Can't Buy My Love managed to sell over 680,000 copies. Due to the success of Can't Buy My Love, Yui's previous album, From Me to You charted once again, adding another 9,000 copies to the Oricon counting.
I Loved Yesterday (2007–2008)
Yui released her 9th single "My Generation/Understand" on June 13, 2007. It was her first double A-side single. "My Generation" was selected as the ending theme for the drama Seito Shokun!, and "Understand" was chosen as the theme song for the movie Dog in a Sidecar (サイドカーに犬), (starring her senpai at Stardust Promotion, Yūko Takeuchi). The single charted at number 1 on the Oricon Weekly Charts the first week of its release.My Short Stories/Holidays in the Sun (2008–present)
Yui released "Summer Song" on July 2, 2008, again charting at number 1 on Oricon Weekly Charts. "Summer Song" sold 83,440 copies in a week, making "Summer Song" second highest in first week sales after "Love & Truth."
Yui released her B-side compilation album, My Short Stories, released 12 November 2008, again charted at number 1 on Oricon. The album included all B-side tracks of all her singles to date, along with new song "I'll Be". Yui is the second female artist whose B-side compilation album topped the charts, after Seiko Matsuda's "Touch Me" in 1984. The last compilation album that reached No. 1 was Mr. Children's B-Side in May 2007. On the success of "My Short Stories", sales of her first album From Me to You rose again.Yui released her new single "Gloria" on January 20, 2010. It was another chart-topper, selling 80,000 copies in the first week. Her next single, "To Mother," was released on June 2, 2010. After two years and three months since I Loved Yesterday, Yui released her new album Holidays in the Sun on July 14, 2010, which includes her latest five singles, from "Summer Song" to "To Mother".