General correspondence includes Daniel Putnam Brinley and Kathrine Sanger Brinley's personal and professional correspondence, mostly incoming letters, for a period of over 75 years. The bulk of the letters are from friends and extended family and discuss travel, mutual acquaintances, social events, and other general news. Much of the personal correspondence does not include the sender's last name. Also found are many letters from Katherine's former priest, E. A. Bazett-Jones, who often addressed her as "daughter" in his letters, and other members the religious community in which the Brinley's were active members.

Professional correspondence discusses Daniel Putnam Brinley's art exhibitions, mural and painting commissions, the sale of his artwork, and his involvement in art organizations and events. Also included is Kathrine Sanger Brinley's correspondence with magazine and book publishers such as magazine and Dodd, Mead & Company, regarding the publishing of her articles and travel books. The Brinley's maintained very little correspondence with other artists, but notable correspondents include the artists William Chadwick, Anthony Euwer, Charles H. Davis, Edward Bruce, Esperanza Gabay, Robert Henri (one letter, dated 1914), Hildreth Meiere, Edwin Blashfield, William A. Coffin, Ernest Peixotto, and Hugh Troy, as well as the writer John Erskine. This subseries also includes a small amount of Elizabeth Loder's correspondence with art organizations, galleries, and others regarding the Brinleys.

Researchers should note that additional correspondence can be found in Series 4: Commission Files and Series 5: Organization Files.