This paper describes the design, status, and test program for the Giant Magellan Telescope (GMT) Primary Mirror Subsystem (M1). It consists of the mirror cells, positioning system, support systems, and thermal control system. The seven 8.4m mirror segments are excluded from this paper because they are considered a separate subsystem of the M1 System.<p> </p> The M1 Subsystem leverages heritage design of similar telescope systems; for example, the Magellan telescopes and the Large Binocular Telescope. The M1 Subsystem incorporates pneumatic force actuators, hardpoints, and a thermal control ventilation system. <p> </p>Design developments have been introduced to address the challenging levels of performance and unique requirements needed by the GMT telescope. Imaging goals necessitate an increase in mirror support performance, figure control, and higher-levels of thermal control. Additionally, there are challenges associated with matching and tracking the relative position of the seven mirror segments for mirror phasing. The design of the static support system needs to protect the mirrors from loads transmitted through the structure during an earthquake. Finally, the telescope design with interchangeable off-axis mirror cells necessitate mirror cells and support components that function under any range of gravitational vector orientations<p> </p>. A full-scale Test Cell prototype is being constructed including production versions of mirror cell components to test and validate the M1 subsystem design. A Mirror Simulator will be used with the Test Cell to validate the M1 Control System. Later, a primary mirror segment will be used with the Test Cell to perform optical tests at the University of Arizona.