He studied at Sacred Heart Seminary in Detroit, Saint John"s Provincial Seminary in Plymouth, and the Catholic University of America in Washington, District of Columbia, from where he obtained his Master"s degree in Education.
He was the seventh Bishop of Winona. Harrington was ordained to the priesthood on June 6, 1959, in the Cathedral of the Most Blessed Sacrament, for the Roman Catholic Archdiocese of Detroit. During his priestly ministry, he served as archdiocesan Assistant Superintendent of Schools and pastor of Holy Name Parish in Birmingham.
From 1977 to 1985, he was rector of his alma mater of Sacred Heart Seminary.
He later became director of the Department of Formation, and pastor of Saint Rene Goupil Parish in Sterling Heights in 1984. On November 23, 1993, Harrington was appointed Auxiliary Bishop of Detroit and Titular Bishop of Uzalis by Pope John Paul World War II He received his episcopal consecration on January 6, 1994 from Archbishop Adam Maida, with Bishops Dale Melczek and Walter Schoenherr serving as co-consecrators.
Harrington chose as his episcopal motto: "Have life more abundantly" (John 10:10). As an auxiliary to Maida, he served as Vicar of the Office of Pastoral Services for Parishes in the archdiocesan curia, Episcopal Liaison for the Pope John Paul II Cultural Center, and regional bishop for Macomb and Saint Clair Counties.
Pope John Paul II later named Bishop Harrington the seventh Bishop of Winona, Minnesota, on November 4, 1998.
He was formally installed as Winona"s ordinary on the fifth anniversary of his episcopal consecration, January 6, 1999. Within the United States Conference of Catholic Bishops, Bishop Harrington chaired the Committee on Education and sat on the Ad Hoc Committee on Catholic Bishops and Catholic Politicians. In these offices, he has expressed his belief that Catholic politicians should oppose abortion.
On October 15, 2008, Pope Benedict XVI named John M. Quinn as Coadjutor Bishop of Winona.
This is a rare appointment as normally Coadjutor Bishops are usually appointed to dioceses where the ordinary is less than the retirement age. On May 7, 2009, Pope Benedict XVI accepted the resignation of Bishop Harrington.
He is also a member of the Priestly Life and Ministry Committee and its liaison to the Subcommittee for Lay Ministry.