A system and method are disclosed for providing a modular user input device such as a computer mouse. A core module can house the sensor, microcontroller and RF modules (if any). It may or may not also house the microswitches and scroll wheel. The core module includes a case module connector that will allow it to accept mouse button inputs from an interchangeable case module, power from a battery pack (if required for wireless operation) and control lights or any other input/output features on the mouse itself. The core module can also accept the connection to the host computer, either through the case module connector or directly via USB or similar industry standard connector.