Past research has shown that the ageing process of bitumen is not solely a function of the bitumen type itself, but rather, added effects have been identified attributed to the presence of fillers. Nevertheless, contradicting results exist between the various studies with respect to the implications of fillers on the ageing process of bitumen. The present study aims to develop a better understanding regarding the role of the fillers in the ageing process of bitumen. Bitumen-filler blends (mastics) were prepared, according to a single design protocol, and the resulting materials along with pure bitumen were subjected to accelerated laboratory ageing by means of the pressure ageing vessel. The ageing effect was studied by using dynamic shear tests and infrared spectroscopy measurements. The results revealed the ability of fillers to mitigate age-hardening of bitumen, despite the fact that formation of carbonyls in the bitumen matrix was observed. Oxygen diffusion and adsorption mechanisms were found to be responsible for the observed favourable effect of fillers on the ageing of bitumen, while the gradation, the mineralogical/elemental composition and, to some extent, the specific surface area of the fillers where established as the prevailing fillers’ properties regulating the oxidation mechanisms. The investigation of recovered binders from aged mastics did not lead to any further insight regarding the effect of fillers on the ageing of bitumen. Instead, features were revealed which manifest that the extraction and recovery of bitumen may not be a suitable approach to address the research question of this study.