He was a Mouseketeer
Johnny Crawford (Born March 26, 1946).
John Ernest Crawford is an American actor, singer, and musician. He first performed before a national audience as a Mouseketeer. At age 12, Crawford rose to prominence playing Mark McCain in the ABC Western series The Rifleman, being nominated for an Emmy Award for Best Supporting Actor at age 13 for his work on the show.
Crawford makes a guest appearance on The Donna Reed Show in “A Very Bright Boy” (Season 4, Episode 15).