Americium was discovered in 1944 by the American scientists Glenn T. Seaborg, Ralph A. James, Leon O. Morgan and Albert Ghiorso. They produced americium by bombarding plutonium-239, an isotope of plutonium, with high energy neutrons. This formed plutonium-240, which was itself bombarded with neutrons. The plutonium-240 changed into plutonium-241, which then decayed into americium-241 through beta decay. This work was carried out at the University of Chicago's Metallurgical Laboratory, now known as Argonne National Laboratory.

Americium's most stable isotope, americium-243, has a half-life of about 7,370 years. It decays into neptunium-239 through alpha decay.

Americium can be produced in kilogram quantities and has a few practical uses. It is used in smoke detectors and can be used as a portable source of gamma rays. Americium-241, with a half-life of 432.2 years, is used in these products because it is easier to produce relatively pure samples of this isotope.