Series I. Missionaries' Papers contains the journals and letters written to the Society from individual missionaries in the Society's employment. The series is arranged alphabetically and the inclusive dates of the materials contained can be found alongside the missionary's name. A chronological list of missionaries which was published by the Society can be found in the collection register, at the front of box one. For a list showing the locations where the individual missionaries served, see Appendix I.

Of all the missionaries, John Sergeant's journals and letters, 1792-1824, are the most complete and informative. John Sergeant served at the Stockbridge Indian Mission for over fifty years until his death in 1824, and his papers gave a minutely detailed account of his trials, accomplishments, and progressions working with the Stockbridge Native Americans. Other missionaries who served Native American communities whose reports may be of particular interest are: Curtis Coe, who reported on the Native Americans in Rhode Island; Gideon Hawley, who served the Mashpees; Timothy Alden, who worked with the Native Americans in Pennsylvania and western New York between 1817 and 1820; Frederick Baylies, who directed a Native American school and mission on Martha's Vineyard from 1818 to 1823; Samuel May, who served the Onondagas and Oneidas at Syracuse, New York from 1847 to 1864; Nettie Stray, Sylvia Joslin, and Hattie Clark who worked with the Seneca Native American school on the Cattaraugus Reservation in New York. Also of note are the letters of Eleazar Williams, a mixed-race, Native American minister who served the Oneida Indians at Green Bay, Wisconsin, and the St. Regis Native Americans in New York and who was ultimately replaced by Rufus King, because his work was found to be unsatisfactory to the Society.

Most of the missionaries employed during the early years of the Society served in poor, sparsely populated white settlements in Maine. Of these, the journals of John Sawyer, Nathaniel Porter, Stetson Raymond, Freeman Parker, Josiah Peet, Daniel Oliver, Elijah Kellogg, Nathan Douglas, and Jonathan Calif seem to be the most informative and representative. Many of the reports from the Maine missionaries discuss not merely the state of religion in the area, but they also detail some of the conflicts with other missionary groups, particularly the Baptists and Methodists. Also of interest are those missionaries who served the Isles of Shoals off the Maine coast; Josiah Steven, Daniel Lovejoy, and Enoch Whipple each spent brief periods on the islands before 1810, and each complained of the lack of "religious zeal" found among the inhabitants. Stevens even comments that much of his time was spent acting as a judge, sentencing several people to sit in the "publick stocks." No missionary was sent to the Isles for the next twenty years, but those who followed after 1840, namely Abraham Plumer, and his son John Plumer, Leavitt Thaxter, George Beebe, and William Hewes, found conditions more acceptable, and their comments are quite revealing concerning the state of life on the islands. Other missionaries who might be of interest are D. W. Stevens, who operated a Seaman's Free Reading Room and Library on Martha's Vineyard; and Hiram Wilson, who operated a mission along the Welland Ship Canal, and who assisted runaway slaves, and slave refugees prior to, and during the Civil War.