This study presents a novel approach that combines thermogravimetric analysis with time-of-flight mass spectrometry (TG-TOFMS), principal component analysis (PCA), and Kendrick mass defect (KMD) analysis─referred to as TG-PCA-KMD─to investigate molecular-scale structural changes and quantitatively assess the progression of thermo-oxidative degradation in glass fiber reinforced polypropylene (GF/PP). TG-TOFMS enables the simultaneous and sensitive detection of both structural changes due to thermo-oxidative degradation and compositional changes in the filler and matrix. PCA and KMD analysis are crucial for identifying specific ion series derived from the degraded PP matrix in the high-resolution mass spectra obtained through TG-TOFMS. Additionally, PCA fitting was employed to selectively extract information on the degraded components of GF/PP from differential thermogravimetric profiles. Our findings demonstrate the advantages and utility of TG-PCA-KMD in the degradation analysis of composite materials.