A cargo box extension assembly for a cargo box of a vehicle and method of operating the extension assembly. The assembly includes a center panel configured and arranged for pivotally connecting to a front surface of a tailgate of the cargo box; a left panel; and a right panel, each of the left and right panels being rotatable between an open position and a closed position where the left and right panels are aligned with each other, when the cargo box extension assembly is installed on the tailgate of the vehicle and the center panel is pivotally connected to the front surface of the tailgate: the cargo box extension assembly being selectively movable between a collapsed position and an extended position; in the collapsed position, the cargo box extension assembly being at least partially received in a recess of the tailgate.