Vigorous ecumenical activity on the part of PCUSA leaders led to this merger, something of a reunion of two long-separated branches of the larger Presbyterian family deriving from the British Isles.

[2] By the time of the merger, the PCUSA had churches in all 50 states, while the heaviest concentration of UPCNA congregations could be found in Western Pennsylvania and parts of Ohio.

Prior to 1967, the ordination vows required an affirmative to this question: "Do you sincerely receive and adopt the Confession of Faith and Catechisms of this Church as containing the system of doctrine taught in Holy Scripture?"

"[5] Despite strong opposition from conservative evangelicals, much of which dovetailed with their hostility toward the denomination's perceived focus on social action that the Confession of 1967 in particular appeared to endorse, nine-tenths of the presbyteries approved the new documents.

Eugene Carson Blake, who served as the stated clerk of the UPCUSA from 1954 until 1966, was particularly active in the civil rights movement, including partaking in the August 28, 1963 March on Washington for Jobs and Freedom, with Martin Luther King Jr.[6] The following year, the UPCUSA took an unprecedented step in electing Edler Garnet Hawkins (1908-1977), an African American, who had served as pastor of St. Augustine Presbyterian Church, as moderator, a position which he filled until 1970.

However, after lobbying from the Presbyterians United for Biblical Concerns, the majority report was rejected by the General Assembly, which voted overwhelmingly to affirm instead that “unrepentant homosexual practice does not accord with the requirements of ordination as set forth in the Book of Church Order.

"[15] Another controversy rocked the UPCUSA when the National Capital Union presbytery voted to receive a minister by the name of Mansfield Kaseman, a move that was upheld by the 1981 General Assembly.

This case resulted in a further wave of departures from the UPCUSA, including those who founded the Evangelical Presbyterian Church, along with longtime Pittsburgh Seminary professor John Gerstner.

[20] Prominent leaders and theologians from the period included Eugene Carson Blake, Robert McAfee Brown, Lloyd John Ogilvie, William Sloane Coffin, and David H. C. Read.