An in situ polymerization method to prepare thermoplastic polyurethanes (TPU)/graphene and TPU/graphite composites have been studied to clarify the influence of intrinsic filler properties and dispersion level on the morphology and properties of the TPU. Two types of graphene, one high-aspect ratio and one low, and graphite flake at various loadings were studied. Rotational rheology experiments on the suspensions of filler in polyol were used to characterize the agglomeration level of the filler after the dispersive mixing step and an effective aspect ratio reflective of the dispersion level was obtained. Variations in the glass transition temperature and thermal transitions of the TPU composites were related to the agglomeration level and intrinsic filler properties.