An under mattress support extends the longevity of an existing mattress that may include concave areas resulting in sagging or soft areas, including a plurality of parallel oriented spaced-apart resilient elongated slat members interconnected by pliable strips to form a parallel slat array. The pliable strip spacing of the parallel resilient slats allows for lateral compression to enhance support. In use, the under mattress support is interposed between a mattress and underlying box spring such that the elongated slats extend along their longitudinal direction from a portion of one side of the mattress to the opposite side of the mattress where the concave portion of a sagging or soft mattress appears during the normal course of use.