An advanced broadband dielectric relaxation spectroscopy technique was developed to measure dipolar reorientation dynamics in an actively deforming amorphous polymer below the glass transition temperature. The application of a weak oscillating electric field during deformation allows for direct probing chain segment mobility. Results show that the application of a monotonically increasing strain on a glassy poly(vinyl chloride) induces a significant increase of the out-of-phase dielectric perrnittivity, thereby reflecting enhancement of the transition activity of chain segments. Moreover, the strain-dependent relaxation spectra suggest that the bifurcation of the relaxation processes decreases together with an overall increase in molecular mobility upon active deformation. The dielectric results also display a highly pronounced sensitivity to the apparent strain rate used for the mechanical excitation and isothermal aging prior to the deformation.