In 1807 he joined the major seminary at Chambery. He was fervent in prayer, and a hard worker with thoughtful regard for his fellow students. He was ordained priest on 21 March 1813 at the age of twenty-three-and-half. As a young priest he was a tireless worker. He taught the little ones by day and continued his theological studies at night. After three years he was appointed to teach at the College of Melan and to serve as Prefect of Discipline. The Archbishop of Chambery appointed him Parish Priest of Le Chaterlard in 1819, at the age of thirty.
Fr. Mermier was an austere priest of unbounded zeal. Mgr. de Thiollez, Bishop of Annecy, appointed him spiritual director at the major seminary in 1823. But in 1826 the Bishop permitted him to dedicate himself entirely to the mission. Gradually a small group of missioners was formed. In the meantime Fr. Mermier sensed the irreplaceable role of parish missions, the need for a religious congregation of missioners and the patronage of St. Francis de Sales.
Fr. Mermier founded the congregation of the Missionaries of St. Francis de Sales (MSFS) for parish mission, foreign mission and the education of the youth. His missionary zeal impelled him to request the Holy See for a mission abroad and to accept a vast mission territory in India when the Congregation was but in its infancy with just eleven professed members. As regards education, he believed that one had to be a mother to the pupil by tenderness, and a father by prudence. He accepted the management of the College of Avian in 1856 and the College of Melan in the following year.
Between 1828 and 1857, Fr. Mermier himself conducted ninety missions. He considered preaching sermon as the chief means of proclaiming the word of God. He adopted the method of carefully prepared simple preaching. He spoke with the conviction of faith, using a fatherly tone of voice marked with a kindly understanding of sinners like St. Francis de Sales had done. He guided the missionaries to lead a life of pleasing and kindly charity in their apostolic ministry and interpersonal relationships. He considered prayer to Our Lady of Seven Dolors to be an eminent Salesian devotional practice.
Despite his advancing age, Fr. Mermier took over the Parish of Pougny as priest-in-charge on 26 June 1857. Even at this time he retained a lively and curious mind. His last years were a time of purification and edification. He fell seriously ill at Pougny and was taken to La Feuillette. His eye-sight and thinking capacity weakened. When he felt a little better he went on a pilgrimage to the shrine of Our Lady of La Salette in July 1859. He suffered a fierce relapse on June 6, 1860 and became totally blind. Meanwhile the Sacred Congregation of Bishops and Regulars approved the Missionaries of St. Francis de Sales as a congregation with simple vows.
On August 10, 1862 Fr. Peter Marie Mermier had a fall, which caused a double fracture of his right leg. He left for his heavenly reward on 30 September 1862.