The 'Pieces of Me' hitmaker marked a decade and a half since the release of 'Autobiography', which was unveiled on July 20, 2004.
She wrote on Instagram: "I can’t believe it’s been 15 years!! (sic)"
Whilst her sister Jessica commented underneath the post: "Me neither!!! (sic)"
Back in 2016, Ashlee revealed she is working on new music with her husband Evan Ross and has been really enjoying the process.
She explained: "We're working on an album together. So in the new year, that will be happening! It's been really fun to work together. This way we can tour together! I'm definitely excited. I love working with my husband. We're doing this album together and then we'll go off and do our own."
Evan also loves working with his wife as it gives the couple an opportunity to "hang out".
He shared: "When you work separately, you don't get to hang out - so this works out well."
Despite having not released a solo album since 'Bittersweet World' in 2008, Evan claimed that Ashlee's passion for music has never subsided.
He said: "I don't think she ever really stopped. Music is something so important to both of us. So we'll release this album and then she'll work on hers."
Ashlee and Evan collaborated on a song together back in 2015 called 'Permission to Love'.
He said at the time: "We did an amazing song together, and we’re trying to figure out what to do with it because I feel like it’s got its own feel to it ... It’s a real powerful song and we’re going to keep making music together. We love doing that. It would only make sense because we sing together all the time. It’s unbelievable how talented Ashlee is, and her writing is amazing. She just has such an ear, and I know she’s working on music now and she’s going to be deciding how she wants to put it out, but yeah, she’s incredible."