Christine Hemming, 53, stole four-month-old Beauty a few days after separating from Liberal Democrat John Hemming last September.
Cameras at the home of Mr Hemming’s mistress Emily Cox filmed Mrs Hemming crawling on her hands and knees beneath a window before entering the property and emerging with the cat under her arm.
During a trial at Birmingham Crown Court, Mrs Hemming accepted it looked “terrible” but said she had simply been trying to avoid being seen by any children and claimed she was there to drop off some post.
She had denied the charge and told jurors she had “no recollection” of taking the tabby, which was never seen again.
But the court heard that in police interviews the mother-of-three admitted taking the cat, telling officers that when she saw the kitten at her love rival’s home she thought, “not only has he replaced me, he’s replaced our cats”.
Giving evidence, Mrs Hemming said the incident at in Moseley, Birmingham, was “just a blur”. “I left holding a kitten – there is no doubt about that – but I have no recollection of picking the kitten up.”
The jury of six men and six women heard that Mrs Hemming, her husband, and Ms Cox, were involved in a “love triangle”.
Mrs Hemming told the court she became aware of her husband’s affair with Ms Cox in 2004 but only learned that he had a “love child” with his mistress from members of the media in April 2005.
Her relationship with her husband had begun to deteriorate because he had been “open and honest” about other relationships.
“I recall I had said that there were about 26 different people with whom he had had liaisons, but at this point we did have quite severe arguments about what he was doing.”
The couple had agreed the MP for Birmingham Yardley could continue to “see some women, including Emily” but that he would always return to the marital home at night.
But after Ms Cox’s daughter was born, the court heard that Mr Hemming began to divide his time between his marital home and that of Ms Cox.
Mrs Hemming, from Moseley, Birmingham, was bailed to appear at the same court for sentencing on October 28.