Milislav Demerec was born on January 11, 1895 in Kostajnica, Croatia. He became interested in genetics while breeding plants at the Krizevci Experiment Station after graduating from the College of Agriculture, Krizevci in 1916. Following the First World War, he attended the College of Agriculture in Grignon, France. In 1919, he became a student of R.A. Emerson at the Department of Genetics at Cornell University, where he earned his Ph.D. in 1923.

Demerec was introduced to Drosophila (fruit fly) genetics by C.W. Metz, then at the Carnegie Institution's Department of Genetics at Cold Spring Harbor Laboratory. In 1923, he was recruited by Charles Davenport to CSHL, and became an investigator in the Department of Genetics.

Demerec's research was characterized by turning to new organisms and tools if they offered him better opportunities for genetic analysis. When he first arrived at CSHL, Demerec analyzed unstable mutations in Drosophila virilis and Delphinium ajacis. Later, in the 1930s, he turned to the study of mutations induced by X rays in Drosophila melanogaster. In the late 1940s, he worked in the field of bacterial genetics, especially fine structure analysis of the bacterial chromosome using transduction.

Demerec had a great aptitude for organization and he became Assistant Director at CSHL from 1936-1941. From 1943 to 1960, he served as Director of the Biological Laboratory and the Carnegie Institution's Department of Genetics, which made up CSHL. During this period, genetics became well established as a major field of research at the Laboratory. Cold Spring Harbor subsequently played a crucial role in promoting the fledgling field of molecular biology.

Two of his most significant recruits to CSHL were Barbara McClintock and Alfred Hershey. He was also skilled in recognizing new research of great potential, a skill put to good use in selecting topics for the annual Symposia. Demerec was an enthusiastic supporter of phage genetics, developed initially by Max Delbrück and Salvador Luria, who began collaboration at Cold Spring Harbor in the summer of 1941. In 1945 they began a series of courses that promoted genetic analysis using phage. The Phage Course continued through Demerec's tenure as Director, training many key players in molecular biology.

Demerec made significant contributions to genetics through the Drosophila Information Service (DIS) which he founded with Calvin Bridges in 1934. The DIS served as an informal source of information on strains and other resources for Drosophila geneticists. Demerec was also the founding editor of Advances in Genetics, which began in 1947, and for twenty years was the only source for reviews of topics in modem genetics.

Following his tenure at CSHL, Demerec became a Research Professor at C. W Post College, Long Island University. Among many honors, Demerec was elected a Member of the National Academy of Sciences in 1946 and a Member of the American Philosophical Society in 1952. He played a major role in the Genetics Society of America, notably as President in 1939. Demerec died on April 12, 1966.