M1 Infantry Helmet with marine corps camouflage cover. One strap missing. Soldier wrote his name in rear strap: Bob Gay.

The M1 is a combination of two "one-size-fits-all" helmets—an outer metal shell, sometimes called the "steel pot", and a hard hat–type liner nestled inside it featuring an adjustable suspension system. Helmet covers and netting would be applied by covering the steel shell with the extra material tucked inside the shell and secured by inserting the liner.

The outer shell should not be worn by itself. The liner can be worn by itself, providing protection similar to a hard hat, and was often worn in such fashion by military policemen, Assistant Drill Instructors, and rifle/machine gun/pistol range staff, although they were supposed to wear steel at the range. The liner is sometimes worn in U.S. military ceremonies and parades, painted white or chromed. The depth of the helmet is 7 inches (180 mm), the width is 9.5 inches (240 mm), and length is 11 inches (280 mm), the steel shell thickness is 0.044" (1.12 mm), The weight of a World War II–era M1 is approximately 3 pounds (1.4 kg), including the liner and chinstrap.

The liner is a hard hat-like support for the suspension, and is designed to fit snugly inside the steel shell.

The first liners were produced in June 1941 and designed by Hawley Products Company. The suspension was initially made from strips of silver rayon webbing stretched around and across the inside of the liner. A sweatband is clipped onto these, and is adjusted to fit around the head of the wearer. Three triangular bands of rayon meet at the top of the helmet, where they were adjusted by a shoestring to fit the height and shape of the wearer's head. A snap-on nape strap cushioned the liner against the back of the wearer's neck and stops it from falling off. As the rayon had a tendency to stretch and not recover its shape, the suspension material was later changed to olive drab number 3, and then olive drab number 7, herringbone twill cotton webbing.

World War II and Korean War-era liners have their own chinstrap made from brown leather. The liner chinstrap does not have loops like the shell; it was either riveted directly to the inside of the liner (early examples) or snapped onto studs. It can still swivel inside the liner. The chinstrap is usually seen looped over the brim of the shell, and helps to keep it in place when its own chinstraps are not in use.

Early liners were made from a mix of compressed paper fibers impregnated with phenolic resin, with olive drab cotton twill fabric stretched over the outside. They were discontinued in November 1942 because they degraded quickly in high heat and high humidity environments. They were replaced by evolving plastic liners, using a process developed by the Inland Division of General Motors. These liners were made of strips of cotton cloth bathed in phenolic resin and draped in a star shape over a liner-shaped mold, where they were subjected to pressure to form a liner. The initial "low pressure" process was deemed unacceptable by the Army, but accepted out of need. These liners were made by St. Clair Manufacturing and Hood Rubber Company. Hawley, Hood, and St. Clair's contracts were cancelled by early 1944, when a "high pressure" process which produced better-quality liners became commercially viable. Companies which produced "high pressure" liners during World War II included Westinghouse Electric & Manufacturing Company, Firestone Tire and Rubber Company, CAPAC Manufacturing, Inland (whose molds were acquired by Firestone after their contract was cancelled), Mine Safety Appliances Company, Seaman Paper Company, and International Molded Plastics, Inc.

Liners essentially identical in construction to "high pressure" World War II examples were produced between 1951 and 1954 during the Korean War by the Micarta Division of Westinghouse and CAPAC Manufacturing. In the 1960s, the M1 helmet liner was redesigned, eliminating the leather chinstrap, nape strap, and changing the suspension webbing to a pattern resembling an asterisk in a coarse cotton web material in lieu of the earlier cotton herringbone twill. In the early 1970s, suspension materials changed to a thicker, more flexible nylon with a rougher unbeveled rim. Later changes included a move to a yellow and green material for liner construction