A ring binder mechanism for holding loose-leaf pages has a housing having a cavity formed in the underside of a central portion of the housing. A pair of hinge plates are disposed between the sides for pivoting movement relative to the housing to open and close rings of the mechanism. The mechanism has an actuator moveable relative to the housing for opening and closing the rings. The actuator has a body, an opening arm extending from the body, a closing arm extending from the body, and a locking finger extending from the body. The closing arm extending into a space between the hinge plates and the central portion of the housing. The hinge plates extend between the opening and closing arms. The locking finger extends into the cavity in the central portion of the housing when the actuator is in the closed position.