A motion assist hinge mechanism includes a first and second hinge part mounted to the tailgate and vehicle body, respectively. The first hinge part includes a first cam having a sloped engagement surface engaged with a sloped engagement surface of a second cam of the second hinge part, under the bias of a resilient member, for moving axially along a pivot axis. The second cam is fixed to the vehicle body against pivotal movement relative to the pivot axis. When the tailgate is moved from a lowered, open position towards a raised, closed position, the resilient member translates a torque assisting force to the tailgate. The surface of the second cam may also include a detent that mates with the surface of the first cam to lock the tailgate at an angle.