Americium was prepared for the first time in 1944/45, by Seaborg, James, Morgan and Ghiorso, through the neutron irradiation of plutonium samples. These researchers discovered the elements 95 and 96 while plutonium was being produced in industrial proportions, in Hanford, to be used in nuclear weapons. Their labor lasted more than one year, being the difficulties so big that one of them suggested that the newly found elements should be called "pandemonium" and "delirium". In spite of this, the elements were given the names americium (in analogy with europium) and curium, honoring Marie and Pierre Curie.

This discovery was announced in a very peculiar way. When asked by students, in a radio show, in November 1945, Seaborg pointed out that during the war, research and development in nuclear weapons had took place, leading to the discovery of two new elements. Nowadays we know that this was the truth, but it was difficult to the youngsters who ran the radio station to convince their teachers that Seaborg was not joking.