In 1910, she became the first president of the Junior League of Brooklyn. In support of national efforts during World War I, she directed YMCA sponsored servicemen's canteens in the New York metropolitan area and worked with the Women's Land Army of America supplying women farm laborers. During World War II, she again aided the war effort as director of the New York City Defense Recreation Committee. Mrs. Pratt was involved with numerous other charitable endeavors including;
Mrs. Pratt served on White House advisory committees on furnishings during the presidential terms of Calvin Coolidge through to Harry S. Truman. In 1925 she was appointed as chair of the White House's first committee, by President Coolidge. Through Pratt's efforts, Eleanor Roosevelt agreed to the establishment of the Subcommittee upon Furniture and Furnishings and Gifts for State Rooms of the White House to be placed under the United States Commission of Fine Arts. Mrs. Pratt served as the subcommittee's chair and as a member until 1947. The curator of the Harriet Barnes Pratt papers notes that,
Horticultural activities
The Pratt estate in Glen Cove, Long Island, Welwyn, was nationally known for its gardens. The estate is now , part of the Nassau County park system. In keeping with the recommendation of Sir Francis Bacon that "...there ought to be Gardens for all the Months of the Year....", Welwyn contained the elements and atmosphere Bacon described. While in some instances professional help was used to lay out individual gardens, most of the planning was done by Mr. and Mrs. Pratt. He would survey the location and she would design the layout and plantings.
1939 World's Fair
This personal, hands on gardening stood her in great stead as she became involved in the national gardening scene. It was, perhaps, her involvement with the 1939 New York World's Fair that brought her the greatest attention for her horticultural activities. Mrs. Pratt had conceived and created Gardens on Parade, a 5-acre area described in the Herald Tribune as "...the Most Stupendous, Most Magnificent, Most Gorgeous exhibition of flowers, shrubs and other horticultural beauties ever assembled." Pratt sold the idea for the Garden to the Fair officials, solicited involvement of individuals and organizations and did most of the fund raising. Laid out in the center of the International Section of the Fair, Gardens on Parade was "...a mammoth flower show, with the flowers growing in their natural, out-of-door element." More than fifty individual gardens were included. These covered the gamut of styles from the formality of the rose and boxwood gardens, to the naturalness of woodland, rock and perennial gardens. Specialty areas featured unusual grasses, espaliered fruit trees and tropical plants. A central, circular installation of weeping cherry trees served as the Havemeyer Memorial, in memory of the late president of the Horticultural Society of New York, Theodore A. Havemeyer. Numerous photos of the gardens can be seen at the 1939 NY World's Fair website
Pratt's longest continual involvement with gardening and horticulture was her involvement with the New York Botanical Garden. This began in 1915 when she first exhibited in a flower show in the Museum building. She was responsible for major developments at the Garden including, Pratt was a member of the Board of Managers of the Garden for over thirty years. In 1964, she became the first person to receive The New York Botanical Garden's Distinguished Service Award for the second time, having first one it in 1951. In September 1944, Pratt donated 75 volumes on gardening and horticulture to the Library of the NYBG. These constitute a named collection within Library's holdings.
