Following her appearance as Roxie Hart in the West End production of Chicago, Simpson announced that she had begun working on her third studio album.

The following year, Simpson joined the main cast of the Melrose Place revamp, which was met with harsh critical reception.

Simpson reprised her role as Roxie Hart for the third time in a 2-dates special production in Hollywood Bowl in July 2013 directed by Brooke Shields.

Both she and Jessica attended Prairie Creek Elementary and North Junior High, located in Richardson, Texas, where the girls were raised.

[17] That same year, Ashlee recorded a song titled "Christmas Past, Present, and Future" for the holiday album School's Out!

[27] "Pieces of Me" earned a gold certification from the Recording Industry Association of America (RIAA), denoting 500,000 copies of the single being sold.

After suffering from what Simpson said was vocal cord inflammation caused by acid reflux on the day of the live broadcast, she lost her voice and was unable to complete the final rehearsals.

The vocal track was promptly lowered, but in the meantime, the band also synchronized with the notes of "Pieces of Me" to mitigate the error, causing even more confusion.

Later, Simpson reappeared during the closing credits with the evening's host, Jude Law, saying: "My band started playing the wrong song, and I didn't know what to do, so I thought I'd do a hoedown.

[31][32] Days later, Simpson called music video series Total Request Live and said that she had almost completely lost her voice.

"[32] Despite the negative media attention surrounding the incident, Simpson released "Shadow" as the second single from Autobiography in the United States.

[35] She also won the Billboard Award for New Female Artist of the Year in December,[36] and in the same month, Entertainment Weekly named her one of its Breakout Stars of 2004.

The album featured similar pop-rock themes as found on her debut effort, though Simpson hoped to incorporate the feel of music from the 1980s into her sound.

[17] The album's lead single, "Boyfriend", was written and composed about the rumors between Simpson and Valderama, and it became her second top-twenty hit in the United States.

[17] In mid-December, Simpson collapsed after performing in Japan and was briefly hospitalized, consequently canceling an appearance at the Radio Music Awards.

[47] In a September 2007 interview, her father, Joe Simpson, explained of the surgery Ashlee had undergone: "There was a real problem with her breathing and that was cured.

"[48] In the middle of 2006, Simpson gave an interview to Marie Claire magazine, in which she was said to have "had it with Hollywood's twisted view of feminine beauty" and was photographed painting a pro-female mural with a group of underprivileged girls from Los Angeles' Green Dot Public School.

[49] By the time the magazine hit newsstands, Simpson had already had her nose job, and some Marie Claire readers complained that this was hypocritical.

[56] Following her appearance as Roxie Hart, Simpson confirmed that she had begun working on her third studio album, initially slated for release in late 2007.

"[58] Hoping to transition to a new sound, Simpson enlisted producers Timbaland and The Neptunes to provide "beat-oriented" songs for the project.

[61] Simpson launched a collection of tops in partnership with the clothing retailer Wet Seal on April 22, 2008, the same day Bittersweet World was released in the United States.

[84] In June 2012, it was announced that Simpson was filming Pawn Shop Chronicles in Baton Rouge, Louisiana, a comedy movie released in 2013.

[84] In 2018, Simpson and Evan Ross teamed up with French brand Zadig & Voltaire and designed a unisex clothing collection titled "Jagger Snow".

[94] That same year, Simpson voiced Strelka in the animated movie Space Dogs Adventure to the Moon, alongside Alicia Silverstone.

[95] Simpson also confirmed to Us Weekly that new music would be released in 2019, which appeared to suggest she was working on her upcoming fourth studio album after an eleven-year hiatus.

[104] She has appeared on several international editions of Cosmopolitan, such as Georgia, Serbia, Russia, Italy, Greece, Ukraine, Slovenia, Netherlands, US and Australia.

[107] With the release of Autobiography, Simpson instantaneously became marketed as an "anti-Britney", drawing comparisons to artists such as Avril Lavigne and Fefe Dobson.

[110][111] With the success of Jessica's reality television series Newlyweds, the sisters became household names in the media and became the focus of much attention.

[112][113][114] She cites her childhood as being the time when she began to sing and admits that she dreamed of being on Broadway and did not expect to break into the pop music scene.

[118] She has cited Guns N' Roses, Gwen Stefani, No Doubt, Joan Jett, The Runaways, Madonna, Pat Benatar, Green Day, Alanis Morissette, Fiona Apple, Chrissie Hynde, and Debbie Harry from Blondie as her musical influences.