Delbrück studied astrophysics, shifting towards theoretical physics, at the University of Göttingen. After receiving his Ph.D., he traveled through England, Denmark, and Switzerland. He met Wolfgang Pauli and Niels Bohr, who got him interested in biology. Delbrück went back to Berlin in 1932 as an assistant to Lise Meitner.
In 1937, he moved to the United States, taking up research at Caltech on Drosophila genetics. Delbrück stayed in the US during World War II, teaching physics at the Vanderbilt University in Nashville. In 1941, he married Mary Bruce, with who he had four children.
In 1942, he and Salvador Luria demonstrate that bacterial resistance to virus infection is caused by random mutation and not adaptive change. For that, they are awarded the Nobel Prize in Physiology or Medicine in 1969, sharing it with Alfred Hershey.