Large optical mirrors for space applications require accurate surface evaluation between successive corrective machining steps. Without proper compensation for self-weight, gravitational deformation can affect the measured surface figure and thereby reduce the reliability of the machining process. In this study, an air-cylinder-based gravity compensation device was developed to maintain the target support loads for large optical mirrors. A proportional–integral–derivative (PID) controller with load-cell feedback was implemented, and the initial gains obtained using the Ziegler–Nichols method were retuned on the basis of the measured system response. Single-axis experiments showed that the retuned controller met the specified performance requirements for overshoot, settling time, and steady-state error, even when applied to air cylinders with different spring reaction forces. The validated control method was then applied, via a sequential multi-channel control scheme, to a five-axis gravity compensation system to assess its applicability to multi-point gravity compensation. The experimental results confirmed stable convergence to the target support loads and effective load regulation at steady state, demonstrating that the proposed system is suitable for the multi-point gravity compensation of large optical mirrors.