At fifteen, Sullivan signed to Jive Records. She recorded an album, which was never released, and she was eventually dropped from the label. Sullivan provided vocals for Kindred the Family Soul's song, "I Am", as well as background vocals on the song "Party's Over", and the title track to their 2003 debut Surrender to Love, and it was during this session she first met rapper Missy Elliott, who would go on to produce both the majority of her debut album, Fearless, and tracks on Love Me Back. Sullivan wrote and recorded with producers Cool & Dre titled "Say I" was given to Dre's then-girlfriend Christina Milian for her third album, So Amazin'. The song became the lead single, peaking at number 13 on the Hot R&B/Hip-Hop Songs, as well as number 21 on the Hot 100. Sullivan's breakthrough song, "Need U Bad", was released in May 2008. The song, which featured additional vocals by Missy Elliott and Sandy "Pepa" Denton of Salt-n-Pepa, later debuted at number 37 on the Billboard Hot 100 and spent a total of 34 weeks atop of the Hot R&B/Hip Hop Songs altogether.
Sullivan's debut album Fearless was released on September 23, 2008. At the time, she wrote and composed many of its songs and served as the album's executive producer alongside Missy Elliott, Salaam Remi and Peter Edge. The album received production from Elliott, Remi, Stargate, Carvin & Ivan, Jack Splash, and Fisticuffs. Fearless debuted at number 1 on the Top R&B/Hip Hop Albums and at number 6 on the Billboard 200. Sullivan followed her début single with the release of the second single from Fearless, "Bust Your Windows", which reached number 4 on the Hot R&B/Hip-Hop Songs and number 31 on the Billboard Hot 100, becoming her most successful single on that chart to date. "Bust Your Windows" appeared on the debut episode of the Fox hit television show Glee and was also nominated for a Grammy for Best R&B Song. In 2014, Stevie Wonder claimed he considered "Bust Your Windows" a classic song. "Lions, Tigers & Bears" was released as the album's third single in December 2008. It scored her third consecutive top ten on the Hot R&B/Hip-Hop Songs, reaching number 10, and garnered some mainstream success by reaching number 74 on the Billboard Hot 100. Sullivan later pursued success in the United Kingdom when she first released "Dream Big" as a single in the UK on February 2009. The song was later released as the fourth and final single from the album in April 2009. The album has a gold certification by the Recording Industry Association of America, and has sold more than 510,000 copies in the United States.
Sullivan was featured on a song entitled "Smoking Gun" with Jadakiss on his third studio album The Last Kiss. She was also a supporting act for fellow American R&B singer Maxwell on his 2008 US tour and was the opening act for Ne-Yo's 2009 Year of the Gentleman Tour. She additionally appeared on Ace Hood's second studio album, Ruthless, on the song "Champion". She went on to headline a few dates with Ryan Leslie, before participating in the Essence Music Festival in June 2009. That same year, Sullivan appeared in commercials for Cotton Incorporated. Additionally, Sullivan made a guest appearance on Snoop Dogg's tenth studio album Malice n Wonderland on the song "Different Languages".