In this study, poly(mandelate-co-glycolate) (PMG), a modified polyglycolide (PGL), is prepared by ring-opening polymerization (ROP) of L-3-phenyl-1,4-dioxane-2,5-dione (PDD); the cyclic dimer of biobased mandelic acid and glycolic acid. The resulting polymer shows an increased glass transition temperature (T ) due to the incorporation of phenyl groups in the chain. High molecular weight PMG is obtained by bulk ROP at 150 °C, and it exhibits a glassy amorphous state with enhanced thermal properties such as a T being 35 °C higher than conventional PGL. PDD is also copolymerized with glycolide (GL) and lactide (LA), resulting in poly(mandelate-co-glycolate/glycolate) ((P(MG/GL)) with GL and poly(mandelate-co-glycolate/lactide) ((P(MG/LA)) with LA. The thermal properties of P(MG/GL) and P(MG/LA) are found to be distinctly different from PMG and conventional PGL and polylactide, and they are tunable with a changing molar ratio of PDD, GL, and LA. Therefore, PDD opens an elegant way to control and tailor the properties of biobased polyesters.