Amine-telechelic poly(tetramethylene oxide) (PTMO) was trimethylated to a quaternary ammonium (QA)-capped PTMO as a bivalent organic macrocation. This QA-capped PTMO (QAPTMO) was used for intercalation of montmorillonite (MMT) platelets with 69% ion exchange yield and 4.03 nm d-spacing. Then, segmented polyurethanes (SPU) with PTMO soft segments were synthesized in the presence of the QAPTMO-intercalated MMT filler in different contents. According to the X-ray and SEM analyses, the polyaddition reaction could result in a full exfoliation of the clay platelets. Two model composites including SPU polymer loaded by unmodified MMT and tetradecyltrimethylammonium bromide (TTAB)-intercalated MMT were also synthesized analogously. The SPU/QAPTMO-MMT composites possessing 5.0 and 7.0 wt.% of the filler obviously showed a better thermal behavior in comparison with SPU/TTAB-MMT and SPU/Na-MMT composites. In addition, a significant increase in the storage moduli of the polyurethane matrices occurred due to the QAPTMO-MMT particles, particularly in the 5.0 wt.% loading content of the filler.