Gary Burghoff ( born May 24 , 1943 ) is an American actor , artist , and musician . He is probably best known for his role as Walter `` Radar '' O'Reilly a company clerck that had extraordinary hearing in the 1970 movie , M \* A \* S \* H and on the television series of the same name . Gary Richard Burghoff ( born May 24 , 1943 ) is an American actor , known for playing the character Corporal Walter Eugene `` Radar '' O'Reilly in the M \* A \* S \* H series and Charlie Brown in the 1967 Off-Broadway musical You 're a Good Man , Charlie Brown .