The mask was made on the fly just before shooting out of cloth, tape, string and paint.
According to producer Anthony Hinds (in Wayne Kinsey's book "Hammer: The Bray Studio Years"), Cary Grant was originally slated not for the role of the Phantom, as is commonly assumed, but for the romantic lead, eventually played by Edward de Souza.
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.
A subplot involving a pair of Scotland Yard police inspectors on the trail of the Phantom was shot especially for the American TV version (by the TV companies, not by Hammer). This was a regular occurrence in this era, most notably with Hammer's films The Kiss of the Vampire (1963) and The Evil of Frankenstein (1964).
The music that Herbert Lom plays as the Phantom is the same music he played as Captain Nemo in Mysterious Island (1961).
The film was originally written for Cary Grant, who wanted to do a horror film. The Phantom's character was rewritten as a more tragic figure, with the dwarf (played here by Ian Wilson) doing the actual violence, to suit Grant's image. Grant declined the part (possibly unhappy with the watered down character) and it went to Lom.