On February 9, 1950, S. G. Thompson, A. Ghiorso, K. Street and G. T. Seaborg performed a careful ion exchange separation of the products formed by irradiation of microgram amounts of Cm 242 with 35 MeV helium ions produced by the 60-inch cyclotron at the University of California, Berkeley. A new radioactive isotope was found, which eluted just ahead of berkelium, in the position expected for element 98. This activity, was assigned to an isotope with mass number 245. The new element was named californium for the state in which it was discovered.

A sufficient amount of californium had to become available to carry out the first experiment with the pure element in concentrated form. B. B. Cunnigham and Thompson, working at the University of California's Radiation Laboratory in Berkeley, concentrated about 0,1 micrograms of californium for a measurement of the magnetic susceptibility.

In 1962, J. C. Wallman and Cunningham, determined the crystal structures of two californium compounds.