The First Congregational Church was formed by Reverend Theodore Clapp in 1834. Clapp originally came to New Orleans from Massachusetts in 1822 as pastor of the First Presbyterian Church, but as a result of a disagreement about church doctrine, he was tried for heresy by the Mississippi Presbytery and excommunicated in 1832. Shortly thereafter, he founded the First Congregational Church, taking with him the majority of his former Presbyterian congregation and retaining the use of the church building formerly occupied by that congregation. Located on St. Charles Street, the building had been sold to Judah Touro in 1822 to help pay off accumulated debts; Touro made the building available to the congregation on a 99 year lease. When this building burned in 1851 (beyond the period covered by these records), Touro purchased a small church on St. Charles St. between Julia and Girod Streets and made it available to Dr. Clapp. A new church was completed in 1855. In his Autobiographical Sketches, Dr. Clapp describes the original church:

"On the lower floor there were one hundred and eighteen pews. The galleries were spacious, and capable of accommodating about four hundred persons. Both sides of the galleries contained free seats, which were always filled by strangers. On this account, our place of worship was often called the Strangers' Church."

He adds that

"our church was honored by the attendance of the most respectable strangers during the winter season. The pews were always taken by residents of the city, and there were more applicants than could be accommodated. It was a usual saying among my orthodox friends, that the merchants and planters who came to New Orleans during the healthy months to transact business never left the city without going to 'the American theatre, the French opera, and Parson Clapp's church.'"

After 1853, the church changed its name to the "First Congregational Unitarian Church." Following Clapp's departure from New Orleans in 1856, the church was led by several other pastors but gradually declined. By 1866, it had no permanent pastor and only a few members, and it finally closed during the years following the Civil War. Additional information on Dr. Clapp's church can be found in Parson Clapp of the Strangers' Church of New Orleans, ed. John Duffy, Louisiana State University Studies, Social Science Series No. 7 (Baton Rouge: Louisiana State University Press, 1957).