Séamus Hegarty was born on January 26, 1940, in Kilcar, Co Donegal.
He received his early education in Kilcar National School, Saint Eunan’s College, Letterkenny, and Saint Patrick’s College, Maynooth, which he attended from 1959 to 1966.
He graduated in Celtic Studies and was fluent in Irish, English and German.
He was ordained to the priesthood at Saint Patrick’s College on 19 June 1966. He taught in Saint Eunan’s College and Colaiste na Croise Naofa, Falcarragh, where he later became President (1971–1981).
Prior to his appointment as bishop he served as curate in Stranorlar.
Bishop Séamus was appointed as Bishop of the Diocese of Raphoe on 12 February 1982.
His episcopal ordination took place at Saint Eunan’s Cathedral, Letterkenny, on 28 March 1982.
Following the resignation of Bishop Edward Daly on October 26, 1993, Bishop Séamus was appointed Bishop of the Diocese of Derry on October 1, 1994 and was installed at Saint Eugene’s Cathedral on November 6, 1994.
On November 7, 2011, Bishop Séamus issued a statement saying that, having been diagnosed with “a condition which is, unfortunately, irreversible and progressive”, he had submitted his resignation because he was no longer able to fulfil his role as diocesan bishop.
On November 23, 2011 the Holy See announced his resignation had been accepted.