I guess that would be a complicated situation, where if you wanted to explain in full you could say you were born Jewish but converted to Catholicism, or you're Catholic but members of your family converted from Judaism a long time ago, or you're Catholic but you have some Jewish family members - whatever the detailed situation was and whatever the context - for example, if you're talking about food and you want to say that your mother always cooked great Jewish dishes.
But if you wanted to keep it simple I suppose you would just say you were Catholic because you go to a Catholic church, for example - whatever message you want to bring across.
And I think most people who have one Jewish and one Catholic parent will just say they are half Jewish, half Catholic.
By the way, when I was very young, the only Christians I knew were Catholic so I though Christianity and Catholicism were the same thing. I didn't know there were other types of Christianity.