High-resolution imaging requirements for space and astronomical applications have pushed physical aperture dimensions to larger sizes. Especially, the off-axis aspheric mirror segments (typical size ≥200 mm) have become a common requirement for these applications in helping the optics designer to improve the system performance while making the overall system very compact. The fabrication of such large off-axis aspheric mirrors with complex surface shapes is progressively multi-faceted, which can be a challenge using traditional manufacturing methods. The process must be assisted by appropriate metrology which dictates subsequent fabrication steps. This paper discusses two methods used for evaluating a concave ellipsoidal off-axis mirror of size 350 mm × 96 mm. Two independent null correctors, one being the conventional null lens and the other being a computer-generated hologram (CGH), are designed and fabricated. An off-axis aspheric segmented mirror is tested using these two null correctors; the pros and cons of each method are discussed in detail. The mirror parameters (RoC, conic constant and OAD) are evaluated independently, and the results are presented. The concurrence between the results obtained by the two methods is self-consistent in deriving the mirror parameters.