A needle guard device mountable to a pre-filled syringe in its ready-to-fill state. The device includes a device shield interconnected to a lock collar with flexible member and biased to move relative to the lock collar. The lock collar interfaces with the syringe neck to attach the device to the syringe. As the device shield moves proximally, rotation arms of the lock collar interact with angled cutouts in the device shield, causing the device shield to rotate relative to the lock collar and disengaging one or more keys on the device shield from one or more keyways in the lock collar triggering the device shield to move from a first configuration in which the device shield is retractable to expose a syringe sharp to a second configuration in which the device shield is fixedly positioned to cover the syringe sharp.