During World War II, Birch participated in the Manhattan Project, working on the design and development of the gun-type nuclear weapon known as Little Boy.

He oversaw its manufacture, and went to Tinian to supervise its assembly and loading into Enola Gay, the Boeing B-29 Superfortress tasked with dropping the bomb.

In 1952 he demonstrated that Earth's mantle is chiefly composed of silicate minerals, with an inner and outer core of molten iron.

[3] There, he wrote or co-wrote four papers, in French, on topics such as the paramagnetic properties of potassium cyanide, and the magnetic moment of Cu++ ions.

William Zisman, another one of Bridgman's Ph.D. students, was hired as the committee's research associate, but, having little interest in the study of rocks, he resigned in 1932.

The position was then offered to Birch, who had little interest or experience in geology either, but with the advent of the Great Depression, jobs were hard to find, and he accepted.

[8][9] In 1942, during World War II, Birch took a leave of absence from Harvard, in order to work at the Massachusetts Institute of Technology Radiation Laboratory, which was developing radar.

There he joined the Los Alamos Laboratory's Ordnance (O) Division, which was under the command of another Naval officer, Captain William S. Parsons.

This proved to be impractical due to contamination of the reactor-bred plutonium with plutonium-240, and in February 1944, the Division switched its attention to the development of the Little Boy, a smaller device using uranium-235.

His citation read:for exceptionally meritorious conduct in the performance of outstanding services to the Government of the United States in connection with the development of the greatest military weapon of all time, the atomic bomb.

Commander Birch's engineering ability, understanding of all principles involved, professional skill and devotion to duty throughout the development and delivery of the atomic bomb were outstanding and were in keeping with the highest traditions of the United States Naval Service.

[15] Albert Francis Birch is known for his experimental work on the properties of Earth-forming minerals at high pressure and temperature, in 1952 he published a well-known paper in the Journal of Geophysical Research, where he demonstrated that the mantle is chiefly composed of silicate minerals, the upper and lower mantle are separated by a thin transition zone associated with silicate phase transitions, and the inner and outer core are alloys of crystalline and molten iron.

The most famous portion of the paper, however, is a humorous footnote he included in the introduction:[16]: 234 Unwary readers should take warning that ordinary language undergoes modification to a high-pressure form when applied to the interior of the Earth.

[22] Since 1992, the American Geophysical Union's Tectonophysics section has sponsored a Francis Birch Lecture, given at its annual meeting by a noted researcher in this field.