Remove any items sitting on top of the mantel. Lay tarps on the floor around the fireplace to protect the flooring from damage.
Position the flat end of a pry bar between the shelf and the brick, beginning at either end of the mantel. Use a hammer to strike the pry bar and force it between the shelf and the brick.
Lift the pry bar to loosen the mantel from the cleats. Move the pry bar toward the center of the mantel, tap it in place and lift the pry bar. Continue to move the pry bar along the underside of the mantel, tap it in place and lift until the shelf is free from the fireplace.
Place a pry bar along the side of the mantel and tap in place with the hammer if it is a full mantel. Begin at the top edge and pry the mantel off the side of the fireplace. Move the pry bar down the side length of the surround; tap the pry bar in place and pry the side piece off until you've removed the entire piece. Repeat on the opposite side of the fireplace.