Remove any trim from the mantel that may be covering any fasteners. Pry trim up with a chisel or pry bar.
Discover how the mantel is fastened to the brick. Look for concrete screws or nails under the front edge. Check out the sides and the back of the mantel for any additional fasteners.
Unscrew any concrete screws you find, using a screwdriver. Pull out any nails using the claw of a hammer.
Lift the mantel off the brick support ledge. If it does not come off easily, it may be fastened to the bricks with some bonding agent. Pry the mantel off with a pry bar until it separates from the brick. Set the mantel aside.
Place the chisel on the mortar by a brick that was under the mantel. Hit the end of the chisel with a mallet until the mortar cracks. Hit the chisel and move it around the mortar until the brick is loose enough to pry out. Move to the next brick and so on.
Remove bricks in a top-to-bottom pattern. When all the bricks are removed, use the chisel to chip away at any mortar that clings to the fireplace area.