A cartridge loader for inserting cartridges into a gun magazine. The cartridge loader fits onto a person's thumb and more particularly fits onto the distal phalange of a person's thumb. In one embodiment of the invention, the cartridge loader includes a thumb protecting member and a cartridge-pushing member. The thumb-protecting member defines interior and exterior surfaces and an interior open cavity of sufficient volume and shape to accommodate a distal phalange of a person's thumb. The cartridge-pushing member defines opposite first and second ends. The first end of the cartridge-pushing member is attached to the exterior surface of the thumb-protecting member. The second end of the cartridge-pushing member defines, at least in part, a concave surface to suit the exterior shape of a cartridge casing.