Adequate knowledge on the root and root canal morphological variations is essential for successful endodontic treatment. Mandibular molar teeth show considerable variability and complexity in their external and internal radicular morphology that require special attention from dental practitioners to provide the best clinical outcomes for the patients. This report aims to describe two clinical cases of mandibular molar teeth with accessory mesial roots/root canals. The first case demonstrates a successful identification and endodontic management of a three-rooted mandibular second molar with an accessory mesial root, and the second case presents a mandibular first molar with five root canals, in which three separate mesial root canals were identified. With the aid of SEM and radiographic examination, both external and internal radicular morphological analysis were also performed on an extracted mandibular second molar tooth having an apical mesial root bifurcation. In addition, a review on the literature was undertaken to identify the available in vitro and in vivo studies that demonstrated these anatomical aberrations in the mesial root of mandibular molar teeth. Keywords: accessory root, accessory root canal, mandibular molar, middle mesial canal, root, root canal