The "London Opera House" used here is actually the Wimbledon Theatre.
The film flopped on its release, and director Terence Fisher fell out of favor with "Hammer Studios", for whom he was not given another film until 1964.
The mask was made on the fly just before shooting out of cloth, tape, string and paint.
The music that Herbert Lom plays as the Phantom is the same music he played as Captain Nemo in "Die geheimnisvolle Insel (1961)."