Kleinert studied physics from 1960 to 1963 at Technische Hochschule Hannover, and then at several universities in the U.S.A. In 1967 he completed his doctorate at the University of Colorado at Boulder, where he had studied under George Gamow, one of the originators of the Big Bang theory. He has been a professor at Freie Universität Berlin since 1969. In the 1970s and '80s, he collaborated with Richard Feynman at the California Institute of Technology in Pasadena, where he presented the foundations of a new theory of critical exponents.
Hagen Kleinert has contributed to the theories of quarks, melting processes, polymers, liquid crystals, and biomembranes. The mathematical basis of his work is path integrals – a universal computing method that is used in many disciplines of theoretical physics and in the field of financial markets. One of numerous textbooks written by Kleinert is called Path Integrals in Quantum Mechanics, Statistics, Polymer Physics, and Financial Markets. It is considered the standard work on path integrals.