Like the Infiniti G35 coupe and sedan and the FX SUV, the 2006 Infiniti M rides on the company's corporate FM (front-midship) platform, though it has been updated for the M. One of the most unique features of this platform is the rear engine mount that gives way in the event of a severe frontal impact, allowing the engine to move below the body and avoid the passenger cabin. In addition to its safety features, this new platform has been designed to provide increased body stiffness and a longer wheelbase. In the case of the M, that wheelbase has grown by four inches versus the 2004 M45, though the overall length has shrunk by over eight inches. Height has increased by 1.8 inches, width has increased by one inch, and ground clearance has decreased by about half an inch.

Despite the shorter body, almost all interior dimensions have increased when compared to the 2004 M45. Front head room decreased by about two-tenths of an inch, but rear head room and front and rear hip room increased. The greatest gains came in leg room and shoulder room, with 2.1 inches of additional front leg room, 4.8 inches of rear leg room, 3.5 inches of front shoulder room, and 2.8 inches of rear shoulder room. In total, the 2006 Infiniti M gained 14 cubic feet of passenger room. Trunk space increased by more than two cubic feet to 14.9, though that figure falls to 11.9 cubic feet with the optional full-size spare tire.

With the updated platform come new aluminum front subframes that strengthen the body structure, and aluminum door, hood and trunk panels. The liberal use of aluminum helps to keep the curb weight between 3,800 and 4,000 lbs. Thanks to the rearward mounting of the engine in the FM platform, distribution of that weight for most models is 54 percent in the front and 46 percent in the rear for optimal handling. Infiniti foregoes fold-down rear seats for the sake of structural integrity, though there is a lockable rear seat pass-through for longer cargo. The trunk lid is lined and includes an integrated handle, making it easier to close.