He worked at the Argonne National Laboratory in 1964, and then did graduate research at Lawrence Berkeley National Laboratory at the University of California, Berkeley, receiving a Ph.D. in chemistry in 1969.

Bernthal spent 1970 to 1975 as an assistant professor at Michigan State University.

He was promoted to associate professor of chemistry and physics in 1975 and taught at Michigan State University until 1978.

In 1988, President of the United States Ronald Reagan nominated Bernthal as Assistant Secretary of State for Oceans and International Environmental and Scientific Affairs and after Senate confirmation, he held that office until 1990.

In 1990, President George H. W. Bush named Bernthal Deputy Director of the National Science Foundation.