The son of Edouard and Eva (Béland) Mathieu, Monsignor René Thomas Mathieu was born in Sanford, Maine on August 24, 1950, the eighth of nine children. He attended Saint Ignatius Grammar School in Sanford, Our Lady of Lourdes Seminary in Cassadaga, New York, and graduated from Assumption Preparatory School & Assumption College in Worcester, Massachusetts (B.A.). He prepared for the priesthood at the North American College and the Gregorian University in Rome, Italy (S.T.B.).

He was ordained deacon in Rome by Archbishop Paul Marcinkus on April 29, 1976, and served his internship at Saint Francis Parish in Liverpool, England. He was ordained to the priesthood by Bishop Amedée W. Proulx on July 23, 1977 at Saint Ignatius Church in Sanford. He served as associate pastor at Saint Louis Parish in Auburn (1977-79); returned to the Gregorian University for graduate studies (1979-80); served as associate pastor at St. André in Biddeford (1980-82); at Saint Augustine in Augusta (1982-83); served as vice-chancellor of the diocese (1983-89), until he was appointed pastor at Holy Rosary Parish, Caribou in 1989. He later was appointed pastor of Holy Cross Parish, Lewiston in 1993, and transferred to Notre Dame de Lourdes Parish, Saco in 2004.  On July 2, 2007, Bishop Richard Malone named him administrator of St. Joseph, Biddeford, St. André, Biddeford, St. Mary, Biddeford, and Most Holy Trinity, Saco, while remaining pastor of Notre Dame de Lourdes Parish.  On July 1, 2008, when these five parishes merged to become Good Shepherd Parish, Msgr. Mathieu was appointed administrator.  When St. Margaret/St. Luke, Old Orchard Beach and St. Philip, Lyman were merged into Good Shepherd Parish on July 1, 2010, Msgr. Mathieu was appointed pastor.

He was named a Prelate of Honor by Pope John Paul II in 2000.

In addition to his parish duties, Monsignor Mathieu serves as a member of the Presbyteral Council.