The Compact Ion Mass Spectrometer (CIMS) is a highly compact ion mass spectrometer capable of high-mass resolution for low-energy space plasma. CIMS is capable of measuring flux, energy, and mass of ions providing unique measurements of the ionospheric outflow and cold plasma in the magnetosphere. Measurements of the ionospheric outflow and cold-magnetospheric ion population will provide the necessary initial conditions of the ion populations that drive some magnetosphere-ionosphere (MI) coupling processes along with magnetospheric ion composition and dynamics. Simultaneous measurements of the cold and hot magnetospheric ion composition in the reconnection region at the magnetotail would provide clues for the outflowing ions as they journey through the plasmasphere and magnetosphere. These data are critical to advancing our current understanding of MI coupling and are required to answer the long-standing questions regarding ionospheric outflow, the source of magnetospheric mass loading, and the subsequent impact on magnetic reconnection. The CIMS utilizes a laminated collimator to define the field-of-view, a laminated electrostatic analyzer to selectively filter ions based on energy-per-charge, a magnetic sector analyzer to separate ions by mass-per-charge, and a microchannel plate with a position sensitive cross-delay anode assembly to detect the location of the ions on the detector plane. This ion mass spectrometer is a simple, compact, and robust instrument ideal for obtaining low-energy (0.1 eV to 500 eV) ion composition measurements of ionospheric and cold magnetospheric ions. The instrument design has significant mass and volume savings when compared to current state-of-the-art ion mass spectrometers and has the additional advantage of being able to simultaneously measure multiple ion species at given energy-per-charge at 100% duty cycle, thus providing a full energy spectra for individual ion species. The concept and operation are intrinsically simple, and enable ultrafast (<0.1 s) measurement of plasma ion composition to provide an improved understanding of the physical processes that drive the complex ion dynamics in the magnetosphere.