In 1961, Heckler & Koch introduced a new light machine gun based on their roller-delayed blowback system as a companion piece to the G3/HK91 rifle. While the German military was quite happy with its MG3 machine guns, H&K expected that other, smaller nations adopting the G3 would be interested in a support weapon that worked the same way. They were not wrong, and the first adopter was Portugal, in 1968. By 1977, some 20,000 had been sold to more than 20 different nations.

In the early 1970s, a number of improvements were made (including a clubfoot stock and optics mounting points on the receiver) and the HK 21A model was introduced. Additional major improvements in the early 1980s created the HK 21E “export” model. This had a longer receiver, longer barrel, strengthened receiver, improved recoil buffer, and was fully modular, able to be assembled into a model HK11, 21, or 23 (that is, magazine fed 7.62mm and belt fed in either 5.56mm or 7.62mm). Today, the model has been replaced in HK catalogs by the new MG4 and MG5 machine guns, but licensed production of the HK 21 in Portugal and Mexico ensures that the guns will be around for many years to come.