Royer began playing drums professionally in 1978 at the age of 17. He began his career with The Detours and Social Distortion, but then moved on to play for the Adolescents on and off between 1980 and 1987. After the Adolescents' first breakup in 1981, Casey went on to form his own band D.I., where he performed as a lead vocalist for the first time.
When the Adolescents regrouped in 1986, Casey returned to the band but left before the 1987 reunion album Brats in Battalions was recorded. He rejoined them again in 2001–2002 for their 20th Anniversary Tour. Casey also joined former Adolescents bandmates in the band ADZ from 1989–93, after which he again made D.I. his primary focus. He has been the only constant member through many D.I. line-up shifts since the band's inception.
In 2011 Royer almost overdosed while watching TV with his 12-year-old son at home in Newport Beach. After his son went for help, Royer was arrested and charged with child endangerment and being under the influence of a controlled substance. The child endangerment charges were dropped, but he was convicted of the misdemeanor drug charge and sentenced to 90 days in jail and three years of probation.