The M240 has been used by the United States armed forces since the mid 1980s. It is used extensively by infantry, as well as ground vehicles, watercraft, and aircraft. Despite not being the lightest medium machine gun in service, the M240 is highly regarded for reliability, and its standardization among NATO members is also seen as a major advantage.

All variants of the M240 series are fed from disintegrating belts, and are capable of firing most types of 7.62 mm NATO ammunition. All variants of the FN MAG 58, including the M240, can use non-disintegrating belts (following replacement of a few easily-swappable parts). They all share the same basic internal parts, which are also interchangeable, for the most part, with other members of the FN MAG family. There are significant differences in weight and some features among some versions for which this does not apply. The M240 is manufactured by FN Manufacturing, a US-based branch of FN Herstal.

The M240B and M240G (see Variants section) are usually fired from an integrated bipod, the M192 tripod, or a vehicular mount.

Manufactured by Fabrique Nationale, the FN MAG was chosen by the U.S. military for different roles after large world-wide searches and competitions. It has mainly replaced the M60 in most roles, though it replaced some other machine guns in the coaxial position. M60s have remained in use, but they are being slowly phased out and replaced by the newer M240s as they wear out.

The M240 is a belt-fed, gas-operated, air-cooled, crew-served, fixed headspace weapon. Its functionality is demonstrated by its ability to be mounted on the M122A1 tripod, a bipod, on vehicles, or on aircraft. The M60 is still, in some cases, used as a door gun on Army helicopters, however.

It was first adopted by the U.S. Army in 1977, as a coaxial tank gun, and slowly adopted for more applications in the 1980s and 1990s. The M240 and M240E1 were adopted for use on vehicles. This led to further adoption in more uses, especially for the Army and Marine infantry. While possessing many of the same basic characteristics as its predecessor, the durability of the M240 system results in superior reliability when compared to the M60. The M240 actually has a more complex gas system than the M60, but gives better reliability combined with lower maintenance requirements, though this comes at greater manufacturing cost and weight.

Compared to other machine guns, its rating of 26,000 Mean Rounds Between Failure (MRBF) is quite high for its weight—in the 1970s when it was first adopted it achieved about 7,000 MRBF. It is not as reliable as some very heavy older designs, but it is quite reliable for its mass.

The U.S. Army is currently working on reducing the weight of the M240B by between four and seven pounds, in the M240B Weight Reduction Program.

The adoption of the M240 has its origins in the late 1960s/early 1970s as a project to procure a new coaxially mounted 7.62 mm machine gun for tanks to replace the M73 and M219 machine guns then being used. It would go on to be deployed in this role in the 1980s, but was additionally adopted for infantry and other uses. It was deployed in these new roles in the 1990s and 2000s.

As mentioned, during the 1970s the Army was looking for new 7.62 mm machine guns for vehicle/AFV mounts. The 1950s-era M73 had been rather troubled, and the derivative M73E1/M219 was not much of an improvement. A number of designs of the period from various countries were considered; the final two candidates were the M60E2 and the FN MAG. They underwent comprehensive testing alongside the older M219 for comparison.