Williams was discovered in 2003 by managers Dave Steunebrink and Richard Williams who signed the 14-year-old to a production deal. At the time she was writing pop songs with songwriters in Nashville. Williams was introduced to Atlantic Records A&R Tom Storms through Richard Williams' attorneys Jim Zumwalt and Kent Marcus, and then signed to the label by Jason Flom. The label's original plan for their new artist was to make her a solo pop artist but Williams objected to this, saying that she wanted to be part of a band and play alternative rock music.
Atlantic decided to go along with her wishes, and she then formed Paramore with Josh Farro, Zac Farro, and Jeremy Davis. The music of Paramore was originally supposed to come out on Atlantic Records, but the label's marketing department decided it would be better for the image of the band to not have them attached to a huge label. They instead released their music through a "cooler" niche label, Fueled by Ramen. The debut album was released in 2005 when Hayley was only 16 years old.
In 2007, she appeared in the music video for "Kiss Me" by New Found Glory.
In the 2007 Kerrang! Readers' Poll she finished second to Evanescence's Amy Lee in the "Sexiest Female" category, going on to win the first place spot for "Sexiest Female" a year later in the 2008 poll, and again in the 2009 poll. She also appears as a playable character in the video game Guitar Hero World Tour.
Williams wrote and recorded the song "Teenagers", which was featured in the soundtrack for the feature film Jennifer's Body. After the release of "Teenagers", Williams stated that she had no plans to establish herself as a solo artist. In 2010, she appeared on the tracks "Airplanes" and "Airplanes (Part II)" from alternative rapper B.o.B's debut album, B.o.B Presents: The Adventures of Bobby Ray. When it was released as a single, "Airplanes" peaked within the top ten in nineteen countries, including number one peak positions in the United Kingdom and New Zealand.