Cyril Toumanoff was born in Saint Petersburg into a family of the military officer of the Russian army. His father's ancestors came of the princely family of Tumanishvili (Tumanov) from Georgia, whose ancestors had emigrated from their original homeland in Cilician Armenia in the 15th century. This family is on the list of the Georgian princes that was attached to the Treaty of Georgievsk concluded between the Georgian king Erekle II and the Russian empress Catherine II in 1783. On December 6, 1850, the Tumanishvili were officially enrolled on the Russian Empire's list of Georgian princely families as knyaz Tumanov. Toumanoff's mother, Elizabeth Zhdanova, was a descendant of a number of Russian noble families, with genealogical ties with the Western European nobility.
Toumanoff's parents fled the October Revolution in St. Petersburg in 1917. His father joined the White Russian forces during the Russian Civil War, while his mother was put to death by the Bolsheviks. Cyril Toumanoff was initially sheltered by his maternal grandparents in Astrakhan. In 1928, Toumanoff's father was able to escape to the United States, bringing his son with him. Cyril Toumanoff attended the Lennox School which he graduated from in 1931 and went to Harvard. His educators John Coddington and Robert P. Blake helped him secure funding for travel to Brussels to study Armenology under Nicholas Adontz and then to Berlin, where he studied Georgian under Michael Tsereteli. During these years, Toumanoff converted to Roman Catholicism. A breach with his Orthodox Christian father ensued and was only terminated when the two reconciled at the latter's deathbed in 1943.
Toumanoff earned a doctorate from the Georgetown University in 1943 and soon accepted a position there which he held until his retirement as a professor emeritus of history in 1970. Thereafter he moved to Rome. A recognized authority on nobiliary and dynastic questions, Prince Toumanoff was also a Professed Knight of the Sovereign Military Order of Malta, High Historical Consultant, Grand Magistry, and the Grand Prior of Bohemia. Having taken religious orders he became known as Fra Cyril.
Cyril Toumanoff died in 1997 in Rome at the age of 83. He is buried at the chapel of the Knights of Malta at Campo Verano, Rome.