In New York City, in 1996 she released her first album, Imaginaryland, consisting mostly of original a cappella music. In 2000, Petra was struck by a car while crossing a street in Los Angeles; the resulting injuries forced her out of performance for several months. In 2005 she released the home-recorded album , a complete a cappella rendition of The Who Sell Out.The project was suggested to Haden by longtime friend Mike Watt, who also gave her the eight-channel multi-track cassette recorder she used to make it. Funded in part by a Durfee grant and wanting to perform the work live, Haden created a new arrangement of The Who Sell Out for a ten-woman a cappella choir called Petra Haden & The Sellouts. The premiere live performance of the full work occurred in July 2005 at the Ford Amphitheatre as a part of the "sound." concert series. She has also released a self-titled collaboration with jazz guitarist Bill Frisell. She contributed to the recording of ØØ Void by drone metal band Sunn O))) and played on the debut and follow up album of Sunn O))) guitarist Greg Anderson's other doom metal band Goatsnake. The Haden Triplets sang the part of the Fates in Anaïs Mitchell's 2010 release Hadestown on Righteous Babe Records. She is also at work on a collection of songs with Yuka Honda, apparently with contributing performances by Sean Lennon. In 2007, Haden recorded an a cappella cover of Journey's "Don't Stop Believin'" for the compilation albumGuilt by Association Vol. 1. This arrangement of the song is very similar to that used in the Glee version of the song in 2009. She performed on the soundtrack to the film An American Crime, starring Catherine Keener and Ellen Page, which premiered on Showtime in May 2008. In 2009, Toyota commissioned Haden to perform three songs for television commercials for the third-generation Toyota Prius, including an a cappella version of the Bellamy Brothers 1970s song "Let Your Love Flow" and worked as an arranger and vocal coach on The Sing Off on NBC, alongside Deke Sharon. Haden played violin on "Sentimental Tune" by Tegan and Sara, off of their album Sainthood. In 2011, Haden formed the group If By Yes with Yuka Honda, the culmination of an eight-year songwriting collaboration. The band released one album, "Salt on Sea Glass" Other band members included Yuko Araki on drums and Hirotaka “Shimmy” Shimizu on guitar, with contributions from David Byrne and Nels Cline. In 2014 she worked on a collaboration with Jim Bianco in Hooray Matinee. The resulting tracks were released on a number of digital outlets. In 2015, she provided vocals for the rock mockumentary "Gentle & Soft: The Blue Jean Committee Story", the 1st season finale of the IFC series Documentary Now!. Haden's voice was used as the falsetto of Blue Jean Committee bassist Clark Honus. Haden even appeared live with Armisen & Hader as "The Blue Jean Committee" on Late Night With Seth Meyers, with Hader lip-synching to Haden's vocals. Also in 2015, Haden again collaborated with Bill Frisell, providing vocals on his 2016 album When You Wish Upon A Star. In 2016, the song "Easy Street" by The Collapsable Hearts Club, which featured Haden and Jim Bianco, played throughout The Walking Dead episode "The Cell". A few hours after the episode's broadcast, the song rose to number 92 in the UK Singles Chart.
