Hollow polymer microspheres with superior elastic properties, high thermal stability, and energy absorbance capabilities are essential in many applications where shock and vibration need to be mitigated, such as in civil, medical, and defense industries. In this paper, the synthesis, fabrication, and characterization of hollow thermoset microspheres for syntactic polymer foam were studied. The hollow polymer microspheres (HPMs) were made by developing core–shell composites and thermally removing the polystyrene core to yield a polysiloxane shell. The HPMs were embedded into a polydimethylsiloxane (PDMS) matrix to form a polymer syntactic foam. The mechanical energy absorption characteristic of polymer syntactic foams was measured by cyclic uniaxial compression testing following ASTM 575. The engineered compression response was demonstrated by fabricating and testing syntactic foams with different porosities, ranging from a 50 vol% to 70 vol% of HPMs. Through scanning electron microscopy (SEM), we observed that the HPM contributes to the energy absorption of the syntactic foam. Moreover, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) determined the necessity of a profound study to understand the effects of varying HPM synthesis parameters, as well as the syntactic foam fabrication methods. It was shown that the compressive modulus and toughness can be increased by 20% using a 70 vol% of porosity with synthesized HPM syntactic foams over bulk PDMS. We also found that the energy absorbed increased by 540% when using a 50 vol% of porosity with fabricated HPM-PDMS syntactic foams.