A needle guard device mountable to a pre-filled syringe in its ready-to-fill state. The device includes a lock collar and a device shield biased to move relative to the lock collar. The lock collar interfaces with the syringe neck to attach the device to the syringe. With the removal of a needle shield assembly comprising rigid and soft needle shields, the lock collar and device shield are free to move proximally along the syringe neck and interact with a syringe step down area to activate the device. As the device moves proximally, retention arms of the device shield interact with the syringe step down, causing the arms to deflect radially outwards to disengage from the lock collar triggering the device shield to move from a first position to a second position.
