Created alongside the development of the Spartan II Program (and later the Spartan III program), MJOLNIR is the name of a series of Powered Armor suits designed to significantly improve the abilities of its wearer, giving superior physical strength, response time, velocity, and defensive capabilities. Originally designed only to protect its wearer from the projectile weaponry used by Insurrectionists, later variants, including the Mark V and Mark VI, would be equipped with an advanced personal energy shield reverse-engineered from captured Covenant technology. Between the external armor plating and the inner fabric suit is a layer of force-amplifying liquid crystal, used to increase speed, strength and reflexes.

The suit also possesses other features that enhance its wearer's abilities. It has numerous clips, belts, and magnetic holsters for the attachment of additional weapons and ammunition; an advanced Heads-Up-Display linked to sensors in the gloves to detect the type of weapon held, and projecting shield strength, ammunition count, a targeting reticule, waypoints, and other helpful data; a radio uplink for communication; and, with the Mark V and Mark VI variants, a layer of memory crystal capable of supporting a ship-grade artificial intelligence.
Although the earlier attempts at developing high powered-armor produced designs that were impractical for battlefield deployment, this changed as soon as the directive came under Dr. Halsey's MJOLNIR Project. The Mark IV was a radical redesign of the suit taking advantage of many technological innovations. This battle suit was the first to feature full integration with the user's body. The suit plugged directly into the user's nervous system through a standard issue Neural Interface. The suit's many functions utilized the user's thoughts for commands, greatly speeding up the user's movements and reflexes. n both game manuals and in the books it is said that the armor weighs in at 1000 pounds, or a half-ton. However, in Halo 3, the armor floats and drifts in water. It has been stated that the armor has a feature that enables the suit to change its density, that can be activated by the occupant and that automatically activates upon the user going unconscious. This would prevent Spartans from sinking to the bottom of an ocean and have to walk to shore to get out. Some other theories suggest that the hydrostatic gel keeps the suit afloat, or that the shields repel the water.