His career spanned over six decades and almost 300 movie and television roles in a wide variety of genres.

His movie career highlights were roles in Gallant Bess, The Monster of Piedras Blancas, Live Fast, Die Young and The Street is my Beat.

The movie in which he made his last screen appearance, The Naked Monster, was released in 2005, twenty years after his death.

Harmon's most notable TV roles were in Bonanza, The Twilight Zone, Perry Mason (as a police fingerprint/ballistics expert), Star Trek (in the episodes "The City on the Edge of Forever" and "A Piece of The Action"), The Rifleman (as the hotel clerk Eddie Halstead) and Gunsmoke (again as a hotel clerk in S1E15's “Gold Mine” & as town judge in S7E24’s “Coventry”).

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