Linear aliphatic polyesters have been investigated extensively over the last decades. However, the relation between the configuration of the ester groups in the main chain and the mechanical properties is only poorly understood. Therefore, in this work the influence of the composition of these polymers on the morphology, mechanical properties and relaxation processes for a set of random copolymers of ε-caprolactone (CL) and ω-pentadecalactone (PDL) is explored. For these isomorphic copolymers, the crystallinity and lamellar thickness was shown to be independent of the composition over the largest part of the composition range. However, the yield stress does decrease significantly when comonomers were introduced in either of the homopolymer chains. Dynamic mechanical analysis revealed an additional high-temperature relaxation process for the copolymers, which was attributed to mobility of the crystalline phase (αc-mobility) and is responsible for the decrease in yield stress relative to the homopolymers. The origin of this mobility was related to the stacking of the ester groups in the crystal lamellae, which occurs less regular in copolymers and therefore gives rise to lower energy barriers for defect propagation (responsible for αc-mobility). Furthermore, the yield-kinetics of polypentadecalactone and two copolymers were accurately captured using the Ree–Eyring theory, showing the relation between αc-mobility and the contribution of both interlamellar and intralamellar shear to the yield stress.