A luminaire mounting assembly for mounting a LED light fixture to a pair of support beams. The luminaire mounting assembly includes an elongated plate and a movable plate. The elongated plate includes opposed sides and a flange extending from each of the opposed sides. The side walls and flanges define parallel tracks therebetween. The movable plate includes having a mounting hole and is adapted to slidably engage each of the parallel tracks. A knockout plate spans the mounting hole and is removably connected to the movable plate via a breakable member. Once the movable plate is suitably positioned and the mounting assembly is fastened to the support beams, the knockout plate is removed and the mounting hole is ready to receive the thin LED light fixture.