After the death of his mother, Bernard sought admission into the Cistercian order. Three years later, he was sent to found a new abbey at an isolated clearing in a glen known as the Val d'Absinthe, about 15 km southeast of Bar-sur-Aube. According to tradition, Bernard founded the monastery on 25 June 1115, naming it Claire Vallée, which evolved into Clairvaux. There Bernard would preach an immediate faith, in which the intercessor was the Virgin Mary. In the year 1128, Bernard assisted at the Council of Troyes, at which he traced the outlines of the Rule of the Knights Templar, who soon became the ideal of Christian nobility.
On the death of Pope Honorius II, Bernard was chosen to judge between rivals for Pope. In 1139, Bernard assisted at the Second Council of the Lateran, where Innocent III was elected Pope. In 1145, Bernard saw one of his disciples, Bernard of Pisa (Eugene III), elected Pope. Having previously helped end the schism within the Church, Bernard was now called upon to combat heresy. In June 1145, Bernard traveled in Southern France and his preaching there helped strengthen support against heresy.
The Pope commissioned Bernard to preach the Second Crusade. The last years of Bernard's life were saddened by the failure of the crusaders, the entire responsibility for which was thrown upon him. Bernard considered it his duty to send an apology to the pope and it is inserted in the second part of his "Book of Consideration". There he explains how, with the crusaders as with the Hebrew people, in whose favor the Lord had multiplies his prodigies, their sins were the cause of their misfortune and miseries. Bernard died at age 63, on August 20, 1153, after 40 years spent in the cloister. He was the first Cistercian monk placed on the calendar of saints, and was canonized by Pope Alexander III on 18 January 1174. Pope Pius VIII bestowed upon him the title "Doctor of the Church". He was buried at the Clairvaux Abbey, but after its dissolution in 1792 by the French revolutionary government, his remains were translated to the Troyes Cathedral.
Bernard's theology and Mariology continue to be of major importance, particularly within the Cistercian and Trappist orders. Bernard led to the foundation of 163 monasteries in different parts of Europe. At his death, they numbered 343. Saint Bernard's Prayer to the shoulder wound of Jesus is often published in Catholic prayer books. His views on the Virgin Mary also influenced other saints, e.g. in the classic text on Mariology, The Glories of Mary, Saint Alphonsus Liguori based his analysis of Mary as the "Gate to Heaven" on Saint Bernard's statement: “No one can enter Heaven unless by Mary, as though through a door.” Dante Alighieri's "Divine Comedy" places him as the last guide for Dante, as he travels through the Empyrean. Dante's choice appears to be based on Bernard's contemplative mysticism, his devotion to Mary, and his reputation for eloquence. He is also the attributed author of the poem often translated in English hymnals as "O Sacred Head, Now Wounded." He also wrote many letters, treatises, and sermons.
The feast day of Saint Bernard in the Roman Catholic Church is on August 20. St. Bernard of Clairvaux is the patron saint of Cistercians, Burgundy, beekeepers, candlemakers, Gibraltar, Queens' College, Cambridge, Speyer Cathedral, and the Knights Templar.