Laser ablation ICP-MS (LA-ICP-MS) is a powerful microbeam technique capable of rapid and precise determination for a large spectrum of trace elements at ppm or sub-ppm levels. Micrometer-scale minerals and inclusions are very common in geological materials, for which direct measurement is restricted by the spot size using LA-ICP-MS (generally 20–50 μm). In this study, ilmenite lamellae intergrown with magnetite were selected as an example to describe a practical algorithm that applies regression analysis to extract the chemical compositions of binary phases from mixed LA-ICP-MS signals. The method accuracy is confirmed by the agreement between the regressed value for various trace elements in ilmenite exsolutions and their reference values (direct analyses using EPMA and LA-ICP-MS). Results were obtained for most detectable components (Mg, Mn, V, Nb, Ta, Sc, Zr, Hf, Sn, et c.) and their relative deviations are within ±10%, even for those <10 ppm (such as Hf and W). Relative standard errors on the regressed value were calculated to evaluate the precision of the method, which is mostly within 10%, and the worst up to 25%. Therefore, the algorithm described in this contribution provides a solution for precise determination of trace element compositions for micrometer-scale ilmenite lamellae in titanomagnetite using LA-ICP-MS, and is potentially practical for other geological materials.