Most likely the mantle is nailed with finish nails that were countersunk and filled over with putty. If you can slip a flat bar under any part of it you may be able to pry it off. Then, use pliers and pull the nails on through the wood, leaving the putty on the surface. If you try to dirve the nails back they will break little chips out of the wood.
The bad news is, it may be installed with countersunk screws, which means it won't want to turn loose. Just give it a try and see what happens.