He was ordained a deacon in 1934 and a priest in 1936, beginning his ministry as a curate at St Bartholomew's, Clyde Road, Dublin under Canon W.C. Simpson. In 1937 he took a position in
Lincoln Theological College but returned to Dublin in 1939 to become Dean of Residence in
Trinity College Dublin and Chaplain Secretary of the
Church of Ireland College of Education. He was appointed
Dean of Cork in 1952. Consecrated a bishop, he served as
Bishop of Cork, Cloyne and Ross, between 1952 and 1956. At forty-two, he was the youngest
Church of Ireland clergyman appointed to a bishopric since
John Gregg in 1915. He served as
Archbishop of Dublin, from 1956 to 1969. During this time, he maintained a courteous relationship with
John Charles McQuaid, his Roman Catholic counterpart as
Archbishop of Dublin. From 1969 to 1980, he served as
Archbishop of Armagh. Alongside Cardinal
William Conway, Simms chaired the first official ecumenical meeting between the leaders of Ireland's Protestant Churches and the Catholic Church in Ballymascanlon Hotel,
Dundalk,
County Louth on 26 September 1973, an important meeting amidst the increasing violence in Northern Ireland. The meeting was protested by
Ian Paisley. ==Scholarly work==