Son of Albert Gailord Hart.
Hart was born in Clarksville (now Clark), Mercer County, Pennsylvania. He graduated from Harvard University in 1880. He was a member of Phi Beta Kappa and a classmate and friend of future U.S. President Theodore Roosevelt. He studied at Paris, Berlin and Freiburg, and received his doctorate from Freiburg in 1883. That same year joined the Harvard faculty, serving as instructor in history in 1883-1887, assistant professor in 1887-1897, becoming professor in 1897. He remained on the faculty for 43 years, retiring in 1926. In retirement he continued to write and edit from a room in Widener Library. He maintained a summer home in New Hampshire near Mount Monadnock.