The Mariette (Nichols) van Steenwyk series contain genealogy, correspondence, financial records, and records of the Home for the Friendless. The genealogy materials trace Mariette van Steenwyk's ancestors back to the Revolutionary War. The primary families involved are Averill, Nichols, and Whittlesey. There are also materials that relate specifically to Matilda Averill Nichols, Mariette van Steenwyk's mother, and to Chester Averill. The correspondence is almost entirely personal in nature. It consists of letters between Mariette's parents, Mathilda and David P. Nichols, and from other relatives, especially her brother Starr Nichols. There are two letters dated 1873 from the U.S. Ambassador to Russia that tell of his experience in being presented to the Russian Court. The correspondence dates from 1825-1927 and is in chronological order.

Mariette van Steenwyk's financial records date from 1902, after the death of her husband. They include a record of investments made from 1903-1905, records of another estate of which Mariette was a beneficiary (1906), and income tax records and returns from 1905-1926, with gaps. Materials from the Home for the Friendless are a treasurer's book from 1900-1907, and bills, receipts, and cancelled checks dated 1907-1909. Two miscellaneous items, a mission organization dues book (1896-1898) and an undated address book complete the materials pertaining to Mariette van Steenwyk.