Born in Brno, Moravia, he studied physics in Prague and Vienna. He worked with Hans Bethe, Edward Teller, Rudolf Peierls, Werner Heisenberg, Victor Weisskopf, Enrico Fermi, Niels Bohr, Lev Landau, Edoardo Amaldi, Emilio Segrè, Leon van Hove and many other prominent physicists of his time. His wife, Els Placzek (née Andriesse) was an ex-wife of a physicist Hans von Halban.
Placzek's major domains of scientific work involve a fundamental theory of Raman spectroscopy, molecular spectroscopy in gases and liquids, neutron physics and mathematical physics. Together with Otto Frisch, he suggested a direct experimental proof of nuclear fission. Together with Niels Bohr and others, he was instrumental in clarifying the role of Uranium 235 for the possibility of nuclear chain reaction.
During his stay in Landau's circle in Kharkov around 1937, Placzek witnessed reality of the Stalinistic regime. His first-hand experience from this stay influenced political opinions of his close friends, Robert Oppenheimer and Edward Teller in particular.
Later, Placzek was the only Czech at some leading positions in the Manhattan project, where he worked from 1943-1946 as a member of the British Mission; first in Canada as the leader of a theoretical division at the Montreal Laboratory and then (since May of 1945) in Los Alamos, later replacing his friend Hans Bethe as the leader of the theoretical group. Since 1948, Placzek was a member of the Institute for Advanced Study in Princeton, USA. Placzek's premature death in Zurich was very likely a suicide influenced by his long-time serious illness. Recently, many new facts about George Placzek and his family roots emerged in connection with a Symposium held in Placzek's memory.