Lib Dem MP John Hemming, 51, had been inside sharing a pizza with the four-year-old daughter from his 13-year affair with former parliamentary assistant Ms Cox.
Unknown to him, his wife sneaked into the £212,000 house he bought his lover and then left with the kitten, Beauty.
Mrs Hemming showed no emotion as the jury at Birmingham crown court found her guilty of burglary.
And, freeing her on bail ahead of passing sentence next month, Judge Elizabeth Fisher told her: “Obviously I make no promise about what the sentence may be.”