A graduate of Carnegie Tech, Morrison became interested in physics, which he studied at the University of California, Berkeley, under the supervision of J. Robert Oppenheimer.

During World War II he joined the Manhattan Project's Metallurgical Laboratory at the University of Chicago, where he worked with Eugene Wigner on the design of nuclear reactors.

In 1944 he moved to the Manhattan Project's Los Alamos Laboratory in New Mexico, where he worked with George Kistiakowsky on the development of explosive lenses required to detonate the implosion-type nuclear weapon.

After the war ended, he traveled to Hiroshima as part of the Manhattan Project's mission to assess the damage.

He then entered the University of California, Berkeley, where he earned his PhD in theoretical physics in 1940 under the supervision of J. Robert Oppenheimer,[7] writing his thesis on "Three Problems in Atomic Electrodynamics".

[8] In 1938, Morrison married Emily Kramer, a girl he had known in high school,[8] and a fellow Carnegie Tech graduate.

In December 1942, with World War II raging around the globe, he was recruited by Robert F. Christy to join the Manhattan Project's Metallurgical Laboratory at the University of Chicago in January 1943.

[15] With the work in Chicago winding down in mid-1944, Morrison moved to the Manhattan Project's Los Alamos Laboratory in New Mexico as a group leader.

After the war ended, Morrison and Robert Serber traveled to Hiroshima as part of the Manhattan Project's mission to assess the damage.

He turned down an offer from Ernest O. Lawrence to return to Berkeley, and instead accepted an invitation from Hans Bethe to join him at the physics faculty at Cornell University.

That year, Life magazine included his image in a gallery of "America's 50 most eminent dupes and fellow travellers".

Theodore Paul Wright, the Acting President of Cornell, was put under great pressure from board members and alumni to fire Morrison, but Bethe remained supportive, and Robert R. Wilson declared that Morrison had "demonstrated his patriotism by the distinguished role he played in the wartime development of the atomic bomb.

"[20] Deane Malott, who became president of Cornell in 1951,[22] was much less sympathetic, and instructed Morrison to curtail all activities beyond his academic field.

[26] Morrison co-wrote a paper with Leonard I. Schiff in 1940 in which they calculated the gamma rays emitted by the process of K-electron capture.

In 1954, he published a paper with Bruno Rossi and Stanislaw Olbert in which they explored Enrico Fermi's theory of how cosmic rays travel through the galaxy.

[31][23] In collaboration with Giuseppe Cocconi, Morrison published a paper in 1959 proposing the potential of microwaves in the search for interstellar communications, a component of the modern SETI program.

[34] In 1963, working in collaboration with a student of his, James Felten, Morrison had investigated the effect of inverse Compton scattering, an important source of cosmic x-rays and gamma rays.

He delivered the 1968 Royal Institution Christmas Lectures on Gulliver's Laws: The Physics of Large and Small,[44] and the 1982 Jansky Lectureship before the National Radio Astronomy Observatory.