His parents were Antal Lepár, a postal clerk, and Margit Etelka Anna Rötzer. His father was born in Temerin, and his mother in Budapest. He changed his last name to “Lékai” on February 12, 1934, pursuant to Ministry of the Interior Permit No. 13,875 of 1934. He was the older brother of Imre Ányos Lékai, O.Cist. He entered the Order on August 29, 1934. He took his first vows on August 30, 1935, and his perpetual vows on January 3, 1941. He completed his theological studies at the Order’s college in Zirc. He earned a degree in history and geography from Pázmány Péter University. He was ordained a priest on June 24, 1941, by Bishop Gyula Czapik of Veszprém. He left the country in 1947 and arrived in the United States by plane on October 19, 1947. He pursued further studies at Marquette University in Milwaukee, WI, USA.

A magyar történetírás, 1790–1830. Budapest, Stephaneum, 1942. (doktori disszertáció)
The White Monks: A History of the Cistercian Order. Okauchee, Wisconsin, Our Lady of Spring Bank, 1953.
Les moines blancs: Histoire de l’ordre cistercien. Paris, Editions de Seuil, [1957].
Geschichte und Wirken der weissen Mönche. Köln, Wienand Verlag, 1958.
The Rise of the Cistercian Strict Observance in Seventeenth Century France. Washington, D.C., Catholic University of America Press, [1968].
The Cistercians: Ideals and Reality. [Kent, Ohio], The Kent State University Press, [1977]. Los Cistercienses: Ideales y realidad. Barcelona, Herder, 1987.
I cistercensi: Ideali e realità. Florance, Certosa, Monaci cistercensi, 1989.
The Cistercians: Ideals and Reality, japanese trans. by Gabriel B. Asakura, Tokyo, 1989.
A ciszterciek: eszmény és valóság. Budapest, Szent István Társulat, 1991.
De orde van Cîteaux: Cisterciënsers en Trappisten: Idealen an werkelijkheden. Achel, Abdij, 1980.
Nicolas Cotheret’s Annals of Cîteaux: Outlined from the French Original. Cistercian Studies Kalamazoo, Michigan, Cistercian Publications, 1982.
Ezen kívül 55 tanulmány és számos újságcikk és lexikon-szócikk.