Free-form lenses are commonly used for controlling the ray direction of LEDs, particularly when a uniform lighting distribution for sensor applications is required. We propose a target grid modification (TGM) method based on Snell’s law, the edge ray principle, and a luminous flux mapping technique to facilitate the design of an LED free-form lens with uniform illuminance properties over a rectangular area. In the proposed method, the size of grids on the target plane is adjusted adaptively in such a way as to improve the uniformity of the illuminance distribution. It is shown that for a given illumination specification, the optimal parameters applied in the TGM method can be easily determined using a pattern search algorithm. The validity of the proposed approach is demonstrated both numerically and experimentally. The results show that the TGM method improves the illuminance uniformity from 50 to 86% and yields an LED output efficiency of 90% given a rectangular target plane with dimensions of 2000 × 750 mm2 and a lens-to-target plane distance of 500 mm.