Timlin was born in Scranton, Pennsylvania, to James and Helen Timlin. He received his elementary education at St. John the EvangelistGrade School and . After graduating from Holy Rosary High School, he attended St. Charles College in Catonsville, Maryland and St. Mary's Seminary in Baltimore, before furthering his studies in Rome at the Pontifical North American College and Pontifical Gregorian University in Rome. On July 16, 1951, he was ordained to the priesthood by Archbishop Martin J. O'Connor. After earning his Bachelor of Sacred Theology degree from the Gregorian University, he was appointed as assistant pastor of in Pittston in 1952. Timlin then served as assistant pastor of St. Peter's Cathedral from 1953 to 1966, when he was named assistant chancellor of the Diocese and private secretary to Bishop J. Carroll McCormick. He was raised to the rank of Chaplain to His Holiness on August 3, 1967, and became chancellor of the Diocese on December 15, 1971 and a Prelate of Honor of His Holiness on April 23, 1972. He also served as chairman of the Diocesan Liturgical Commission and the Priests' Education Committee, as well as librarian and secretary of in Dalton. He became a member of the Diocesan Board of Consultors in 1972, and President of the Board of Directors of in 1975. On July 26, 1976, Timlin was appointed Auxiliary Bishop of Scranton and Titular Bishop of Gunugus by Pope Paul VI. He received his episcopal consecration on the following September 21 from Bishop McCormick, with Archbishop John R. Quinn and Bishop Stanley J. Ott serving as co-consecrators, at St. Peter's Cathedral. Serving a five-year term as Episcopal Moderator of the , he became vicar general of the Scranton Diocese in 1976 and pastor of the in 1979. In 1983, he was named chairman of the Board of Advisors of St. Pius X Seminary and of the Preparatory Commission for the Diocesan Synod. Following the promotion of Bishop John J. O'Connor to Archbishop of New York, Pope John Paul II appointed Timlin to succeed him as the eighth Bishop of Scranton on April 24, 1984. Installed on June 7 of that year, he was the first native son of Scranton to become its diocesan bishop. During his tenure, he held the Second Diocesan Synod, established the "Bishop's Annual Appeal", presided over a major restructuring of parishes due to the priest shortage, and introduced a new policy for Catholic schools consisting of regional mergers, construction of modern facilities, new fund-raising efforts, and a more equitable sharing of operational costs between parents, pastors, and the Diocese. In 2003, Timlin refused to attend the commencement ceremonies for the University of Scranton, because of the allegedly pro-choice views of honorary degree-recipient Chris Matthews. He has been repeatedly accused of ignoring allegations of homosexual and pedophilic tendencies among the diocesan clergy. After reaching the mandatory retirement age of 75, Timlin resigned as Bishop on July 25, 2003, after a nearly 20-year-long tenure. He served as administrator of St. Joseph's Church in Wilkes-Barre from February to July 2004, when he became rector of Villa St. Joseph in Dunmore, the diocesan residence for retired priests.
On August 14, 2018, a grand jury investigation into child sex abuse in the Pennsylvania Catholic Church revealed Timlin paid a family $75,000 to remain silent about a priest, Thomas Skotek, who raped a teenage girl, got her pregnant and arranged for her to get an abortion. Skotek resigned as Pastor of Saint Stanislaus Church, Hazleton. Timlin wrote to the rapist on October 9, 1986: "This is a very difficult time in your life, and I realize how upset you are. I share your grief. With the help of God, who never abandons us and who is always near, when we need him, this too will pass away, and all will be able to pick up and go on living. Please be assured that I am most willing to do whatever I can do to help." After a stay in a Catholic center for psychological evaluation, Skotek was assigned to another parish. He continued his ministry until 2002. The report also indicated that Timlin had made a request to the judge sentencing Robert Caparelli to send him to a church treatment center instead of prison. In light of his failure to protect children, the name of Timlin House at the University of Scranton was removed, and its plaza was renamed Romero Plaza, after Óscar Romero. On August 31, 2018, Scranton Bishop Joseph Bambera forbade Timlin from representing the diocese at all public events, liturgical or otherwise, given Timlin's failure to protect children from abusers. This was the most he could restrict another bishop; however, he also sent the matter to the Vatican Congregation for Bishops, which has jurisdiction over additional aspects of Bishop Timlin’s ministry. Bambera himself had served as the Vicar for Priests for the Diocese of Scranton from 1995 to 1998, and he admitted that during that time he had helped Bishop Timlin reassign a priest who had abused a minor, although the decision was made by Timlin.
