An exemplary method includes virtual set-top box (“STB”) device system receiving a registration request to register a mobile computing device to operate as a virtual STB device, verifying that the mobile computing device is within a predefined vicinity of a dedicated STB device that is located at a user premises, providing, to the mobile computing device, a virtual STB key for use by the mobile computing device to operate as the virtual STB device by leveraging access rights of an end-user account of a media content distribution service to access the media content distributed by way of the media content distribution service, and facilitating sharing of the virtual STB key with a media content processing device for use by the media content processing device to access the media content distributed by way of the media content distribution service. Corresponding systems and methods are also described.