Nigel Bass demanded pounds 10,000 for returning the animal. "It has got to be a straight swap - money for the cat," he told Ilona Szekere, 32, of Clapham, south-west London.
"Taking her cat was - for her - like taking a child," Recorder Ann Curnow said to Bass as she warned him he faced jail.
"Obviously her devotion to her cat is akin to that which other people - fortunate enough to have children - have for them."
The judge remanded Bass for reports after he admitted blackmailing his former colleague and friend last January.
Bass, 33, of Shepherd's Bush, west London, was arrested after Miss Szekere reported the matter to police and a meeting was arranged at which the cat was to be returned in exchange for money. He told police: "I must have been temporarily insane ... It's like a bad bloody dream. The whole thing is a sick joke that went wrong."Reuse content