Large-area mapping is of great valuable in microwave remote sensing, but it is a contradiction between swath width and azimuth resolution due to the minimum antenna area constraint. In this paper, we consider a specific wide-area mapping technique with high-altitude high-speed platform, where range ambiguity suppression is a technical challenge. To resolve this problem, we present a multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) with multiple antennas placed in the cross-track direction. The increased degrees-of-freedom (DOFs) provide a potential to resolve possible range and/or azimuth ambiguities. After formalizing the system scheme and signal model, an iterative matched filtering algorithm is presented, which can efficiently suppress the cross-correlation interferences in the multichannel data separation. Furthermore, a range-Doppler based image formation algorithm is derived. The MIMO SAR system performance is evaluated by the range-ambiguity-to-signal ratio (RASR) performance. Numerical simulation results validate the effectiveness of the proposed MIMO SAR in high-altitude high-speed platform SAR for large-area remote sensing.