Lou Caplan was born in Baltimore, Maryland in 1936. He received his baccalaurete degree with honors from Williams College where he was elected to Phi Beta Kappa in his Junior year and his MD degree is from the University of Maryland in 1962, graduating summa cum laude and valedictorian of his medical school class. He took his medical internship and residency at the Boston City Hospital (BCH) followed by two years in the US Army working in internal medicine. He had his Neurology residency at the Harvard Neurological Unit at BCH under Dr Derek Denny-Brown and then was a Stroke Fellow under the tutelage of Dr. C Miller Fisher at the Massachusetts General Hospital.

In the 1970s, Caplan was a staff Neurologist at the Beth Isreal Hospital. There he founded the Harvard Stroke Registry and became Chair of Neurology in 1975. In 1978, he became Neurologist-in-chief at the Michael Reese Hospital in Chicago and Professor of Neurology at the University of Chicago. In 1983, he returned to Boston as the Neurologist-in-chief at the new England Medical Center and Professor and Chair of Neurology at Tufts University School of Medicine. In January of 1998, he returned to the Beth Israel Deaconess Medical Center and Harvard Medical School.

Dr Caplan is board certified both in Neurology and Internal Medicine. He is the author or editor of 35 books mostly about various aspects of stroke, and of over 600 articles in scientific medical journals. He has been on the editorial board of 21 different scientific journals. He has trained 29 American and 28 international Stroke fellows. He has delivered 31 named lectures in the USA and internationally. He received the Gold Medal Alumni award of the University of Maryland School of Medicine in 1992  and delivered the annual Pincoffs lecture at the medical school that same year. Long active in the American Heart Association, he served as the Chairman of the Stroke Council, a member of the Scientific Advisory Council, and received their American Heart Association's Scientific Councils Distinguished Achievement Award in 1993. He has been President of the Boston and Chicago Neurological Societies and has held positions of responsibility in both the American Academy of Neurology and the American Neurological Association.