Shearer was later a member of Los Angeles radio comedy group The Credibility Gap, 1968 - 1974, and regular on Saturday Night Live in 1979 - 1980 and 1984 - 1985 seasons. Shearer co-created, co-wrote and co-starred in Rob Reiner's 1984 film This is Spinal Tap with Michael McKean and Christopher Guest; the three of them also collaborated on the acclaimed 2003 spoof "A Mighty Wind"
Shearer is probably best-known for his astounding voiceover work on The Simpsons (1989 to date), where he does the voices of "Charles Montgomery Burns", "Waylon Smithers", "Ned Flanders", "Reverend Timothy Lovejoy" and Principal "Seymour Skinner", among others. Since 1983 Shearer has been the host of the public radio comedy/music program Le Show.