Simpson described the process of writing the album's twelve songs as being similar to keeping a diary, and before the album's US release she said that "My inspiration came from what I have gone through in the past three years. Every single day I was thinking of what I was going through and would write songs about it." The lyrics primarily deal with love and heartbreak; in particular, much of the album's lyrical content was inspired by Simpson's breakup with her boyfriend at the time, Josh Henderson. She has described the album as being "very honest" and "very true to my emotions"—"I wasn't afraid to say that I was hurting and how I got over it." Simpson worked closely with a number of experienced songwriters on the album. The album's producer, John Shanks—who won a Grammy in February 2005, in part for his work as producer on Autobiography—receives songwriting credit alongside Simpson on all but two songs. Kara DioGuardi also receives songwriting credit, together with Simpson and Shanks, on seven of the songs, including the three singles. (See the track listing.)
The album incorporates rock as well as pop elements, which contrasts with the heavily pop-oriented music of sister, Jessica. One reviewer noted that, unlike Jessica's music, Autobiography "relies on glitzy guitars and big power-pop riffs". On her reality show, Simpson emphasized that she did not want to make her music like pop singers such as her sister or Hilary Duff; instead, she has cited musicians such as Chrissie Hynde and Joan Jett as influences.
In a 2004 interview, Simpson said that when she began seeking a recording contract record labels would not meet with her because they thought she just wanted "to be like her sister". Simpson also said that she did not want to meet with Jessica's record label (Columbia) because she wanted to be signed because of her music, rather than her sister. She eventually signed with Geffen in 2003. As for her role as co-writer of the songs, Simpson has said that she had a lot of input lyrically: "...I come up with the whole concept of the song. To me, writing is a very important thing. It's what I've always done and what I've always loved to do, and it was a big part of my wanting to do a record. So my label was amazing because they really let me have my hands in there. I got to write a lot." Simpson said of making the album: "It's a lot of work. From finding the right label to the actual recording, it took about nine months, then it was followed by the publicity work." In an extensive list of thank-yous in the album's liner notes, Simpson includes Benji and Joel Madden of the band Good Charlotte, with whom she worked on the song "Harder Everyday", which was released as an international bonus track.