With Casey Royer, Agnew played in Social Distortion and the Detours in 1979. He joined the Adolescents in 1980. His guitar style and writing dominated the Adolescents' Blue Album on Frontier Records. Agnew left the band soon after the album's release in 1981 due to personal and creative differences. Agnew released his first solo album, All By Myself, on which he plays all the instruments. He was briefly in the band Christian Death and released Only Theatre of Pain. He joined D.I. in 1983. With D.I., Agnew played on their 1983 EP Team Goon (which used to be known as simply D.I.) as well as their first two albums (1985's Ancient Artifacts and 1986's Horse Bites Dog Cries) before deciding to leave the band in 1987. When the Adolescents reunited in 1986, Agnew rejoined the band until its demise in 1989. Agnew went on to release two more solo albums, Rikk Agnew's Yard Sale and Turtle. He rejoined Royer and Tony Adolescent to record 'Where were you?' with ADZ in 1992.
In 2000, Agnew again regrouped with the Adolescents and continued to play with them until 2003. Agnew toured with the reunited 45 Grave, in place of original guitarist Paul B. Cutler, in 2004 and 2005. He plays guitar in PooP with Scott Hoogland, former frontman of the influential Orange County band The Mechanics. Agnew has played in the reunited Detours. After playing in Christian Death 1334 with James McGearty and EVA O, Agnew joined the new lineup of Voodoo Church as a second guitarist.
As of 2013 he is most active playing in Wrong Beach, producing bands, recording solo work & Various projects.
He became engaged to Goth Diva/Ex Christian Death member Gitane Demone on May 3, 2013.