Mass spectrometry (MS) is a powerful analytical technique that is widely used to characterize a variety of analytes across diverse fields. In the area of biomass conversion, which is essential for producing sustainable materials and energy, the role of MS is pivotal. Biomass conversion processes, such as solvent liquefaction and pyrolysis, generate a wide range of industrially valuable materials including bio-based polymers, fuels, and activated carbons. However, the inherent complexity and heterogeneity of biomass and its transformation products pose significant analytical challenges. Advanced MS techniques, such as GC-MS, LC-MS, ICP-MS and MALDI-MS, are essential for a comprehensive analysis, providing detailed insights into the compositions, impurities, and potential inhibitors that influence process optimization and product quality. This review systematically explores recent advancements in MS-based methods for the analysis of biomass-derived products. We discuss fundamental innovations in biomass conversion processes and highlight the applications of various MS techniques in assessing the chemical complexity of these materials. The insights provided by MS techniques not only facilitate process optimization but also support the sustainable production of high-value materials from biomass, aligning with ongoing efforts to enhance environmental sustainability and resource efficiency.