The son of wool magnate David L. Einstein, Lewis Einstein had two sisters: Amy, who married Joel Elias Spingarn, and Florence, who married Sir Charles Waldstein. He graduated from Columbia University in 1898, and earned a master's degree in 1899.
Einstein's diplomatic career began in 1903, when he was appointed as Third Secretary of Legation at Constantinople. The next year, he married Helen Ralli, a noted English beauty a number of years older than he, whose sister was married to the younger son of Francis Stonor, 4th Baron Camoys. This marriage led to friction between Einstein and his father, who worried that Ralli would damage the younger Einstein's career; Ralli was twice a divorcee, and divorced women could not be received in some European courts.
Einstein advanced from Second Secretary to First Secretary and then Charge d'Affairs during the Young Turk Revolution of 1908, remaining in Constantinople despite the hostilities. He served as United States Ambassador to Costa Rica for one month in 1911, before his wife's ill health in the country's high altitude forced him to leave the post. He returned to Constantinople in 1915 and wrote his diaries which would be later published under the name Inside Constantinople: A Diplomatist's Diary During the Dardanelles. In 1921, Warren Harding made him United States Ambassador to Czechoslovakia, a position he held until 1930. He was also a member of the Council on Foreign Relations.
Lewis Einstein was disinherited by his father after marrying Ralli, except for a sum of $125,000 . After Einstein's death, newspapers reported that a $1,250,000 share of the elder Einstein's estate, valued in total at approximately $4,000,000, had been set aside for Lewis Einstein in the event that he divorced his wife, and that it passed to his sister Lady Waldstein after he declined to do so. This report was denied by Lady Waldstein, who indicated that the father's only wish regarding Lewis Einstein was to see that he was "taken care of", a means she accomplished by granting him an annual allowance of $20,000. Earlier, Lewis had received nothing from the estate of his mother Caroline Einstein, who instead divided her property among Einstein's sisters and various friends among European nobility.
Einstein wrote the following books:
- Inside Constantinople : A Diplomatist's Diary during the Dardanelles Expedition, April–September, 1915 (1918)
- Roosevelt : His Mind in Action (1930)
- Divided Loyalties : Americans in England during the War of Independence (1933)
- A Diplomat Looks Back (1968)
Einstein also engaged in a longtime correspondence with Oliver Wendell Holmes, Jr., and in 1964 their collected letters were published in the volume The Holmes-Einstein Letters : Correspondence of Mr. Justice Holmes and Lewis Einstein 1903-1935, edited by James Bishop Peabody.
- "Lady Waldstein Claims $1,250,000; Names Herself Heir to Fund Trustees Say Was for Her Brother if He Left Wife", The New York Times, December 2, 1913, page 1.
- "Diplomatic Appointments - Lewis Einstein of This City Gets a Secretaryship in the Embassy at Paris", The New York Times, May 20, 1903, page 3.
- "Einstein Demands", Time magazine, August 20, 1928.
- Marconi Transatlantic Wireless Telegraph, "Lewis Einstein in London", The New York Times, February 11, 1912, page C5.
- "Eight Ministers Named By Harding", The New York Times, October 5, 1921, page 14.
- "Lady Waldstein Denies Agreement", The New York Times, December 3, 1913, page 13.
- "Nothing to Lewis Einstein" (scroll down), The New York Times, November 11, 1910, page 6.
- Liebmann, George W. Diplomacy between the Wars: Five Diplomats and the Shaping of the Modern World (London I. B. Tauris, 2008), covers Einstein