Diamond was born in Kishinev, Bessarabia, at the time part of the Russian Empire. His family emigrated to the United States in 1904, following the Kishinev pogrom. He began his medical studies at Harvard University in 1919 and, on graduating in 1923, entered Harvard Medical School, receiving his M.D. in 1927. Shortly after finishing medical school, Diamond studied briefly with Florence Sabin at the Rockefeller Institute before returning to New England, where he spent several years studying pediatrics at Boston Children's Hospital under the guidance of Dr. Kenneth Blackfan.
Diamond set up one of the first pediatric hematology research centers in the United States at Children's. Focusing on anemias, by 1930 he had succeeded in identifying thalassemia, a hereditary anemia that affected children of Italian and Greek ancestry. In 1932, along with Blackfan, he identified erythroblastosis fetalis, later called Hemolytic disease of the newborn, at that time a significant disorder among newborns. He also discovered the blood diseases Gardner-Diamond syndrome, a painful bruising disorder, and Shwachman-Diamond syndrome, a rare genetic disorder that affects many different organs.
- "Louis K. Diamond: 1902-1999". HemOnc Today. 1 January 2008. Retrieved 2010-02-16.