Van Lierde was born in Hasselt, Belgium to a Dutch family. After his education, he joined the Order of Saint Augustine and was ordained as a priest on 30 May 1931. After receiving doctorates in theology and philosophy, he headed the Augustinian College Saint Monica in Rome, where he was hiding many refugees including military officers, Jews and anti-fascist politicians during the war years.
Pope Pius XII named him Titular Bishop of Porphyreon in 1951 and Vicar General of the Vatican State. Van Lierde chose as his episcopal motto: Custodiens veritatem, (Keeping the truth). October 1958 he gave the last rites to the Pope and presided over his funeral in Saint Peter Basilica. He functioned as Sacristan in four Papal Conclaves, 1958, 1963 and twice in 1978. After having served forty years and one day, Pope John Paul II accepted his resignation in 1991 and named him Emeritus Vicar General, with the privilege to maintain his residence inside the Vatican near the papal quarters.
In 1954, when Pope Pius XII seemed to be without hope of survival, he asked for Petrus Canisius van Lierde to anoint him. Miraculously, the Pope recovered and lived another four years. In May 1963, Pope John XXIII was asking for him. On 31 May it had become clear that the cancer had overcome the resistance of the Pope. At 11 A.M. Petrus Canisius van Lierde as Papal Sacristan was at the bedside of the dying pope, ready to anoint him. The Pope begins to speak for a very last time: “I had the great grace to be born into a Christian family, modest and poor, but with the fear of the Lord. …My time on earth is drawing to a close. But Christ lives on and continues his work in the Church. Souls, souls, Ut omnes unum sint, (that all may be one). Van Lierde then anointed his eyes, ears, mouth, hands and feet. Overcome by emotion, he forgot the right order of anointing. Pope John gently helped him. Then the Pope bid him and all the other bystanders a last farewell.
Throughout his life, Van Lierde was an ardent admirer of Pope Pius XII, whose determination and courage to save lives during the German occupation of Rome he repeatedly described. Pius XII possessed a sharp, penetrating intelligence, an extraordinary memory, a methodical way, and strong will to work, a deep love of God and of people, whom he loved through God and because of Him. Pius XII lived simply when alone, and displayed friendly reluctance, when he met strangers. He had instant capacity to understand the other person and showed great tact towards those who may have been very direct. From 1958 to 1995, he presided over the annual liturgical celebrations for Pope Pius XII in the crypt of Saint Peter Basilica. In presence of Cardinal Joseph Ratzinger, he led the funeral services of Madre Pascalina Lehnert in the Vatican's Campo Santo Teutonico. Petrus Canisius J. van Lierde died on 12 March 1995, the anniversary of the coronation of his favourite Pope.