An expandable system for mobile navigation facilitates a modular implementation of structural units to add desired functionality to a base navigation device. The system is embodied as a handheld mobile navigation device in one arrangement, including a base unit housing containing circuitry for determining a geographic location of the navigation device and a module unit housing containing circuitry for delivering additional functional activity. In particular, the base unit housing includes a primary interface for interconnecting with a secondary interface of the module unit housing, to enable signals generated or handled by a circuitry component of the module unit to be relayed to the circuitry of the base unit housing. Additionally, the module unit housing is configured to be releasably attached directly with the base unit housing upon the primary interface and secondary interface interconnecting with one another, to form the mobile navigation device as a physically connected package.