(1948) and Ph.D. (1950) degrees from Harvard University under Julian Schwinger.
In 1955, he became associate professor at the University of Pennsylvania, achieving full professorship in 1958 and retiring in 1994.
Klein studied models of collective behavior in finite many-body systems, especially in nuclear physics, for example in Boson model and in an extension of the Hartree–Fock method with Robert Kerman (Kerman–Klein method).
In 1964, Klein published an article about spontaneous symmetry breaking with his student Benjamin W. Lee and contributed to the appearance of Higgs mechanism.
[8] He was a Sloan Fellow and Guggenheim Fellow, Honorary Doctor of Goethe University of Frankfurt, and Alexander von Humboldt Senior Scientist.