In an embodiment of the invention, an unlocked mobile device is configured to capture images, analyze the images to detect a user's face, and automatically lock the device in response to determining that a user's face does not appear in the images. The camera capturing and face recognition processing may be triggered by the device having detected that it has been motionless for a threshold period of time. In another embodiment, a locked mobile device is configured to capture an initial image using its camera, capture a new image in response to detecting movement of the device, determine that the device moved to a use position, capture a subsequent image in response to determining that the device moved to a use position, analyze the subsequent image to detect a user's face, and unlock the device in response to detecting the user's face. Other embodiments are also described and claimed.