Polarization is a key parameter in light–matter interactions and is consequently closely linked to light manipulation, detection, and analysis. Terahertz (THz) waves, characterized by their broad bandwidth and long wavelength, pose significant challenges to efficient polarization control with existing technologies. Here, we leverage the mesoscale wavelength characteristics of THz waves and employ a mirror-coupled total internal reflection structure to mechanically modulate the phase difference between p-and s-waves by up to 289°. By incorporating a liquid crystal phase shifter to provide adaptive phase compensation, dispersion is eliminated across a broad bandwidth. We demonstrate active switching of orthogonal linear polarizations and handedness-selective quarter-wave conversions in the 1.6–3.4 THz range, achieving an average degree of linear/circular polarization exceeding 0.996. Furthermore, arbitrary polarization at any center frequency is achieved with a fractional bandwidth exceeding 90%. This customizable-bandwidth and multifunctional device offers an accurate and universal polarization control solution for various THz systems, paving the way for numerous polarization-sensitive applications.