She was born January 30, 1912, the Jewish daughter of the banker Maurice Wertheim and his first wife Alma Morgenthau. Her father was an individual of wealth and prestige, the owner of The Nation magazine, president of the American Jewish Congress, prominent art collector, and a founder of the Theatre Guild. Her mother was the daughter of Henry Morgenthau, Sr., Woodrow Wilson's ambassador to the Ottoman Empire.
While she did not explicitly mention it in her book The Guns of August, Tuchman was present for one of the pivotal events of the book: the pursuit of the German battle cruiser Goeben and light cruiser Breslau. In her account of the pursuit she wrote, "That morning [August 10, 1914] there arrived in Constantinople the small Italian passenger steamer which had witnessed the Gloucester's action against Goeben and Breslau. Among its passengers were the daughter, son-in-law and three grandchildren of the American ambassador Mr. Henry Morgenthau." As she was a grandchild of Henry Morgenthau, she is referring to herself, which is confirmed in her later book Practicing History, in which she tells the story of her father, Maurice Wertheim, traveling from Constantinople to Jerusalem on August 29, 1914, to deliver funds to the Jewish community there. Thus, at two, Tuchman was present during the pursuit of Goeben and Breslau, which she documented 48 years later.
Wertheim was influenced at an early age by the books of Lucy Fitch Perkins and G.A. Henty, as well as the historical novels of Alexandre Dumas. She attended the Walden School on Manhattan's Upper West Side. She received her Bachelor of Arts from Radcliffe College in 1933, having studied history and literature.