Integration of multi-chip cascaded multiple-input multiple-output (MIMO) millimeter-wave (mmWave) sensors with synthetic aperture radar (SAR) imaging will enable cost-effective and scalable solutions for a variety of applications including security, automotive, and surveillance. In this paper, the first three-dimensional (3-D) holographic MIMO-SAR imaging system using cascaded mmWave sensors is designed and implemented. The challenges imposed by the use of cascaded mmWave sensors in high-resolution MIMO-SAR imaging systems are discussed. Especially, important signal processing functions such as near-field multistatic image reconstruction suitable for large MIMO apertures, multi-channel array calibration, spatial sampling, and image resolution are presented in the context of 3-D MIMO-SAR imaging. The prototyped 3-D MIMO-SAR imaging system is described in detail, along with various real imaging results.