A graduate of California Institute of Technology, he earned his doctorate from Princeton University in 1933, and joined the Berkeley Radiation Laboratory where he discovered oxygen-15 and beryllium-10.

[1] He had a younger sister, Catherine Helen, whose son John Clauser (that is, McMillan's nephew) won the Nobel Prize in Physics in 2022.

[4] He then took his Doctor of Philosophy from Princeton University in 1933, writing his thesis on the "Deflection of a Beam of HCI Molecules in a Non-Homogeneous Electric Field" under the supervision of Edward Condon.

[5][6] In 1932, McMillan was awarded a National Research Council fellowship, allowing him to attend a university of his choice for postdoctoral study.

[2][8] The main focus of the Radiation laboratory at this time was the development of the cyclotron, and McMillan, who was appointed to the faculty at Berkeley as an instructor in 1935, soon became involved in the effort.

[8] In 1935, McMillan, Lawrence and Robert Thornton carried out cyclotron experiments with deuteron beams that produced a series of unexpected results.

Their experiments indicated a nuclear interaction at lower energies than would be expected from a simple calculation of the Coulomb barrier between a deuteron and a target nucleus.

Berkeley theoretical physicist Robert Oppenheimer and his graduate student Melba Phillips developed the Oppenheimer–Phillips process to explain the phenomenon.

In addition to the nuclear fission products reported by Hahn and Strassmann, they detected two unusual radioactive isotopes, one with a half-life of about 2.3 days, and the other with one of around 23 minutes.

Both scientists began their work using the prevailing theory, but Segrè rapidly determined that McMillan's sample was not at all similar to rhenium.

Instead, when he reacted it with hydrogen fluoride (HF) with a strong oxidizing agent present, it behaved like members of the rare-earth elements.

[13] McMillan realized that his 1939 work with Segrè had failed to test the chemical reactions of the radioactive source with sufficient rigor.

This reaction resulted in the sample precipitating with the HF, an action that definitively ruled out the possibility that the unknown substance was a rare earth.

In May 1940, Philip Abelson from the Carnegie Institute in Washington, DC, who had independently also attempted to separate the isotope with the 2.3-day half-life, visited Berkeley for a short vacation, and they began to collaborate.

As a final step, McMillan and Abelson prepared a much larger sample of bombarded uranium that had a prominent 23-minute half-life from 239U and demonstrated conclusively that the unknown 2.3-day half-life increased in strength in concert with a decrease in the 23-minute activity through the following reaction: This proved that the unknown radioactive source originated from the decay of uranium and, coupled with the previous observation that the source was different chemically from all known elements, proved beyond all doubt that a new element had been discovered.

[15] McMillan suddenly departed for war-related work at this point, leaving Glenn Seaborg to pursue this line of research and discover the second transuranium element, plutonium.

In November 1940, he began working at the MIT Radiation Laboratory in Cambridge, Massachusetts, where he participated in the development and testing of airborne microwave radar during World War II.

[17][21][15] Oppenheimer recruited McMillan to join the Manhattan Project, the wartime effort to create atomic bombs, in September 1942.

[23] He recruited personnel for the laboratory, including Richard Feynman and Robert R. Wilson, established the test area known as the Anchor Ranch, and scoured the country for technical equipment from machine tools to a cyclotron.

[24] As the laboratory took shape, McMillan became deputy head of the gun-type nuclear weapon effort under Navy Captain William S. Parsons, an ordnance expert.

[26] John von Neumann looked at the implosion program in September 1943, and proposed a radical solution involving explosive lenses.

Segrè's group had tested samples of plutonium bred in the Manhattan Project's nuclear reactors and found that it contained quantities of plutonium-240, an isotope that caused spontaneous fission, making Thin Man impractical.

[20] His gold Nobel Prize medal is in the National Museum of American History, a division of The Smithsonian, in Washington DC.