He was born on April 11, 1860 in Shushi. He received his primary education in local diocesan and provincial schools. After the death of his father, not having the opportunity to continue his studies, he began to work.
Leo began his literary works in the late 1870s. He worked for "Mshak", "Ardzagank", "Murch", "Horizon", "Taraz" and other newspapers and magazines for decades. He wrote short stories, and novels, as well as tried his hand at theater.
In 1924, at the invitation of the government of Soviet Armenia, he came from Tbilisi to Yerevan and started working at YSU, where he studied Armenian history. His lectures were published under manuscript rights and together with H. Manandyan's lectures; they became the first university textbooks of Armenian history.
Among with H. Manandyan, M. Abeghyan, Hr. Acharyan and others, Leo also has a great contribution to bringing the teaching of Armenology subjects to a scientific level at YSU.
In 1925, Leo became a professor. In the same year, he became a full member of the Institute of Science and Art of the ASSR (since 1930, the Institute of Sciences of the ASSR).
Leo's contribution to the study of Armenian social thought, history of Armenian literature, and culture is great. He was the first to thoroughly study the life and activities of Stepanos Nazaryants and Grigor Artsruni, outstanding representatives of Armenian public thought, and tried to show their place and role in the arena of social-political and intellectual development.
The most valuable are the historical works of the distinguished scientist. His "History of Armenia" is the result of many years of work, written in simple and attractive language, filled with rich factual material and historical generalizations. The first volume of this extensive work (Tbilisi, 1917) is dedicated to the history of the ancient period of Armenia, from ancient times to the 5th century, the second volume (Yerevan, 1947) describes the history of the 5th-11th centuries, the third volume (Yerevan, 1946) covers the situation of the Armenian people in the 16th-18th centuries.
Leo was one of the first historians who embarked on the study of the history of the new and most recent regions of the Armenian people, issues related to the Armenian liberation movements, the Armenian question, the First World War, and the great tragedy of the Armenian people.
A collection of his works in 10 volumes was published in 1966-1987.
Leo passed away on September 14, 1932, in Yerevan.
The bronze bust of Leo in the lobby of YSU's central building symbolizes the role of an outstanding scientist-pedagogue in the development of university science and education.
