Abstract: An adjustable support assembly for a digitizer device, such as a track ball or mouse, mounts between a computer keyboard support tray and an articulating arm normally attached to the tray for supporting and adjusting the tray. The support assembly includes a right-hand support pad and a left-hand support pad that are slidable underneath the keyboard support tray in a stored position. One of the pads can be selectively exposed for use, while the other pad is stored underneath the keyboard support tray. The upper surface of the support pads are textured to enhance the tracking movement of a mouse ball. The exposed mouse pad is rotatably adjustable about an axis parallel to a longitudinal axis of the keyboard support tray to provide a horizontal surface for the mouse to ride on, even if the keyboard support tray is tilted away from horizontal. The support pads additionally include a slot extending inwardly from an outer edge thereof.