A mechanical actuator for valves and other mechanical devices, according to the present invention, includes a locking mechanism disposed about a rotatable valve-actuating stem that normally maintains the actuator in a locked condition, thereby preventing inadvertent movement of the valve or other mechanical device with which the actuator may be associated. The locking mechanism is unlocked upon predetermined initial rotation of a handwheel or other means for rotating the actuating stem of the mechanical actuator, thereby allowing the actuator to open or close the valve mechanism. Cessation of rotation of the handwheel or overrunning of the actuator that might occur by pressure actuation of the valve will cause automatic relocking of the actuator.