Born in Russian Empire to Jewish parents, he emigrated to the U.S. in 1887 to escape political persecution. Due to the May Laws, he was imprisoned for at least two years, according to William James Sidis' biographer, Amy Wallace. He later credited his ability to think to this long solitary confinement. His wife, Sarah Mandelbaum Sidis, M.D., and her family fled the pogroms about 1889.
Boris completed four degrees at Harvard (a B.A., M.A., Ph.D. and M.D.) and studied under William James. He was influential in the early 20th century, known for pioneering work in psychopathology (founding the New York State Psychopathic Institute and the Journal of Abnormal Psychology), hypnoid/hypnotic states, and group psychology. He is also noted for vigorously applying the Theory of Evolution to the study of psychology.
He vehemently opposed World War I, viewing war as a social disease, and denigrated the widely held concept of eugenics. He sought to provide insight into why people behave as they do, particularly in cases of a mob frenzy or religious mania. With the publication of his book Nervous Ills: Their Cause and Cure in 1922, he summarized much of his previous work in diagnosing, understanding and treating nervous disorders. He saw fear as an underlying cause of much human mental suffering and problematic behavior.
Sidis applied his own psychological approaches to raising his son, William James Sidis, in whom he wished to promote a high intellectual capacity. His son has been considered among the most intelligent people ever (with a ratio IQ broadly estimated at 250–300). After receiving much publicity for his childhood feats, he came to live an eccentric life, and died in relative obscurity. Boris Sidis himself derided intelligence testing as "silly, pedantic, absurd, and grossly misleading."