Yes I can not think of any reason why one might question that it is a mammal. It walks on all four legs and it has fur which to me would suggest mammal and in fact they are not too different than the dog which I know for sure is mammal.
Of course cats are mammals! Who would think that cats are not mammals. They have everything that a mammal would have. Cats have hair on their bodies, they give live birth, they feed they young by giving them milk, and they are warm blooded. If anybody thinks thinks hat cats aren't mammals then they better check AGAIN!
Actually they are reptiles..... NOT. Are you nuts? Seriously this even a question that needs debate? They have fur, they born live babies they feed milk. And that's a proof that its a mammal. You got that? Cats are mammals. P.S I posted on No side to be sarcastic. By person