Dissolved organic matter (DOM) in mature landfill leachate is significantly different from that in young landfill leachate; the composition of DOM greatly influences both biological treatment and advanced treatment processes. In the present study, the transformation and degradation mechanisms of landfill leachates in a combined semi-aerobic aged refuse biofilter (SAARB) and ozonation process was investigated using organic removal analysis, molecular weight distribution (MWD), 3D-EEM-PARAFAC analysis, UV-Vis spectroscopy, and linear regression. Results revealed that the DOM in mature landfill leachate contained a greater amount of aromatic substances and had higher molecular weight than DOM in young landfill leachate. After the SAARB process, humus contained in the SAARB was discharged with effluent from both mature and young landfill leachate. Due to the differences in composition and structure of organic matter, the COD removal efficiency (17.89%) of SAARB effluent from treating mature landfill leachate (mature SAARB effluent) was much lower than that (45.91%) of SAARB effluent from treating young landfill leachate (young SAARB effluent) under the same operational parameters of the ozonation process. As indicated by PARAFAC analysis, better chemical stability of DOM in mature SAARB effluent resulted in inferior ozone treatment efficiency. Furthermore, the hydrophobicity and the concentration of benzene ring compounds in the mature and young SAARB effluent were reduced significantly by the ozonation process. Therefore, great improvements in the biodegradability of SAARB effluents were achieved in the ozonation process. Overall, the results of this study provide suggestions and guidance for practical applications of these technologies.