His accurate measurements of mass differences between nuclear isotopes allowed him to confirm Albert Einstein's mass–energy equivalence concept.

"[2] This marked the beginning of his dedication to ending the testing of nuclear weapons and to efforts to maintain civilian control of future developments in that field.

While at high school he developed an interest in ham radio which inspired him to enter Massachusetts Institute of Technology (MIT) in 1921 to study electrical engineering.

Upon graduating from MIT in 1926, he enrolled at Princeton University, where Karl T. Compton, a consultant to General Electric, was on the faculty.

[8] While at Princeton, Bainbridge created his first mass spectrograph, came up with methods for identifying elements, and started studying nuclei.

[4] In 1929, he was awarded a Ph.D. in his new field, writing his thesis on "A search for element 87 by analysis of positive rays" under the supervision of Henry DeWolf Smyth.

At the time the Franklin Institute's Bartol Research Foundation was located on the Swarthmore College campus in Pennsylvania, and was directed by W. F. G. Swann, an English physicist with an interest in nuclear physics.

[14] Since Bainbridge was the first to successfully test Einstein’s theory of the equivalence of mass and energy, he was awarded the Louis Edward Levy Medal.

[15] In 1933, Bainbridge was awarded a prestigious Guggenheim Fellowship, which he used to travel to England and work at Ernest Rutherford's Cavendish Laboratory at Cambridge University.

[4] When his Guggenheim fellowship expired in September 1934, he returned to the United States, where he accepted an associate professorship at Harvard University.

He proposed using a Holweck pump to produce the vacuum necessary for this work, and enlisted George B. Kistiakowsky and E. Bright Wilson to help.

[10][16] In September 1940, with World War II raging in Europe, the British Tizard Mission brought a number of new technologies to the United States, including a cavity magnetron, a high-powered device that generates microwaves using the interaction of a stream of electrons with a magnetic field.

[20] Bainbridge spent two and a half years at the Massachusetts Institute of Technology’s Radiation laboratory working on radar development.

[24] Bainbridge ended up finding a site that was approximately 200 miles away from Los Alamos, located in the Alamogordo Gunnery Range.

Bainbridge along with his assistant director, John Williams who was also a physicist planned and oversaw the construction of the needed facilities at the test site.

"My personal nightmare", he later wrote, "was knowing that if the bomb didn't go off or hangfired, I, as head of the test, would have to go to the tower first and seek to find out what had gone wrong.

[4] Bainbridge was relieved that the Trinity test had been a success, relating in a 1975 Bulletin of the Atomic Scientists article, "I had a feeling of exhilaration that the 'gadget' had gone off properly followed by one of deep relief.

Utilizing his new device, Bainbridge was able to establish the existence of the neutrino, which is a basic component of matter that had eluded scientists for some time.

[30] During Bainbridge’s remaining years at Harvard, he continued to work towards finding new mechanisms to obtain precise yields of atomic masses.

In 1950 he was one of twelve prominent scientists who petitioned President Harry S. Truman to declare that the United States would never be the first to use the hydrogen bomb.