The papers cover much of the career of this leading Low Churchman. They begin with his transfer to the Diocese of Maryland, ordination as priest by Bishop James Kemp, and first ministry at Christ Church, Georgetown, D.C., 1821-1825. Substantial materials concern the dispute with Bishop Kemp over proposals for a theological seminary in Maryland. A letter of 1822 excommunicates a parishioner for refusing to conduct family worship in his home. Other correspondence of that year concerns McIlvaine's criticisms of the faculty of St. John's College, Annapolis, and his service as chaplain of the U.S. Senate. Further controversies involved attacks on Bishops John Hobart and Henry U. Onderdonk, 1828-1829, and many later debates on matters of business, doctrine, and churchmanship with Bishops Philander Chase and George W. Doane, among others. The majority of the papers relate to affairs of the Diocese of Ohio and of the Episcopal Church in general. They include material on the history of Kenyon College; numerous charges and pastoral letters to the Episcopal Church in Ohio, together with many printed sermons, addresses and tracts produced by McIlvaine. The papers detail his theological beliefs, Low Church principles, evangelical zeal, interest in education, views on church music, opposition to ritualism and the Roman Catholic Church, and involvement with the General Theological Seminary. There is much concerning relations with the Church of England and condemnation of the Tractarian movement. Letters, sermons, and prayers of the Civil War period show McIlvaine's attitude to the war, support for Bishop Whittingham's Unionist stand in Maryland, and views on the separation of the Episcopal Churches in the North and South. After the war comes material on his role the the reunion of the Episcopal Church and his support of the Freedmens' Commission. A letter of 1869 concerns a meeting in Baltimore of the Peabody Fund for Education in the South. Several letters of 1869-1871 deal with McIlvaine's membership in a committee of the House of Bishops on theological education, and other items show his support for the Society for the Promotion of Evangelical Knowledge. References to individual bishops and other clergy abound in these papers.