Fourier-transform infrared (FTIR) spectroscopy and magneto-spectroscopy are powerful tools for probing band structure and charge transport in quantum materials. While these techniques are commonly used at liquid helium temperatures (~4 K), many exotic correlated phenomena—such as fractional quantum Hall states, unconventional superconductivity, and spin liquids—emerge only at ultra-low temperatures (~100 mK) achievable with dilution refrigerators. This work presents a system capable of conducting broadband far- to mid-infrared cyclotron resonance (CR) spectroscopy in high magnetic fields and ultra-low temperatures of ~120 mK.