A major challenge to the scalability of cryogenic computing platforms is the heat load associated with the growing number of electrical cable connections between the superconducting circuitry and the room-temperature environment. Compared with electrical cables, optical fibres have significantly lower thermal conductivity and are widely used in modern telecommunications. However, optical modulation at cryogenic temperatures remains relatively unexplored. Here we report the cryogenic electro-optical readout of a superconducting electromechanical circuit using a commercial titanium-doped lithium niobate modulator. We demonstrate coherent spectroscopy by measuring optomechanically induced transparency and incoherent thermometry by encoding the mechanical sidebands in an optical signal. We also show that our modulators can maintain their room-temperature Pockels coefficient at 800 mK. Further optimization of the modulator design-for example, by using longer waveguides and materials with a higher Pockels coefficient-could reduce the added noise of our setup to similar levels as current semiconductor microwave amplifiers. A commercial titanium-doped lithium niobate phase modulator can be employed at temperatures as low as 800 mK for the electro-optical readout of a superconducting electromechanical circuit at 15 mK.