A downhole magnet tool comprises a top sub having an inner bore terminating in a downhole facing open end, the inner bore defining a centralizer receptacle of smaller width than a width of the downhole facing open end, a mandrel terminating upwardly in a centralizer stem, the centralizer stem being fitted in the centralizer receptacle of the top sub, and a magnet mounted on a downhole facing surface of the mandrel. A method of assembly of a downhole magnet tool comprises mounting a magnet on the mandrel and inserting the centralizer stem into the centralizer receptacle.