The prediction of ultralow magnetic damping in Co2MnZ Heusler half-metal thin-film magnets is explored in this study and the damping response is shown to be linked to the underlying electronic properties. By substitution of the Z elements in high-crystalline-quality films (Co2MnZ with Z = Si, Ge, Sn, Al, Ga, Sb), electronic properties such as the minority-spin band gap, Fermi-energy position in the band gap, and spin polarization can be tuned and the consequences for magnetization dynamics analyzed. The experimental results allow us to directly explore the interplay of spin polarization, spin gap, Fermi-energy position, and the magnetic damping obtained in these films, together with predictions from ab initio calculations. The ultralow magnetic damping coefficients measured in the range from 4.1 × 10-4 to 9 × 10-4 for Co2MnSi, Co2MnGe, Co2MnSn, and Co2MnSb are the lowest values obtained on a conductive layer and offer a clear experimental demonstration of theoretical predictions on half-metal magnetic Heusler compounds and a pathway for future materials design.