The Morton Arboretum was founded in 1922.

The Morton Arboretum was founded in 1922 by Joy Morton (1855-1934). At that time, three focus areas were put into place—the living plant collections, a research library, and a herbarium. The Education Program was added in 1940, a formal Research Program was established, and the Sterling Morton Library was built in 1963.

The inspiration for the Arboretum had its origins in Mr. Morton’s own family tree. His father, J. Sterling Morton (1832–1902), was the founder of the original Arbor Day in 1872, and served as Secretary of Agriculture under President Grover Cleveland.

When Joy Morton died in 1934, Joy's daughter, Jean (Mrs. Joseph M. Cudahy, 1883–1953), became trustee chair, and served in that capacity for nineteen years. Under her leadership, the Arboretum developed a national reputation. In memory of her father, she built the Administration Building, which provided a reception area for visitors, staff offices, and space for the herbarium and library. Clarence Godshalk (1897–1988), who had worked at the Arboretum since 1921, was named director in 1939. Trained as a landscape architect in the naturalistic style of O.C. Simonds and others, he further developed the Arboretum's distinctive landscapes.

To better accommodate visitors, Mrs. Cudahy commissioned a custom-designed open-air bus in 1937, and in 1940 she started the Arboretum's renowned education program by inviting the gifted naturalist and ecologist, May Theilgaard Watts (1890–1975), to teach. In 1942, a new educational facility, the Thornhill Building, was opened on the site of the razed Morton mansion. The innovative public education program Mrs. Watts developed was the first such program among U.S. arboretums. Mrs. Watts became widely known for her Tree Finder and Flower Finder identification booklets; her book Reading the Landscape (1957, revised in 1975 & 1999); and for originating the Illinois Prairie Path in 1963, the first rails-to-trails project in the U.S.

Sterling Morton (1885–1961) became trustee chair after Jean's death in 1953 and served for eight years. Achievements during his tenure included building a new research wing and auditorium onto the Administration Building in memory of his sister; initiating a practical research program; increasing the size of the professional staff; and adding considerable acreage.

After Sterling's death in 1961, his daughter Suzette Morton Davidson (1911–1996) became chairman, serving until 1977. She built the Sterling Morton Library in memory of her father and the first Visitor Center in memory of her mother. In the ensuing years, she greatly expanded the rare botanical book and art collection.

In 1966, after nearly 45 years at the Arboretum, Clarence Godshalk retired. The trustees, in selecting a new Director, wanted to bring research up to a level of excellence comparable to the existing programs in collections and education. They selected Dr. Marion Trufant Hall (1920– ), a scientist of broad interests and experience, as Director. Over the next 24 years he transformed the Arboretum into a major scientific and cultural institution.

The year 1972, the 50th anniversary of the Arboretum, was a critical one. A general master plan was developed; the Visitor Center was built in memory of Suzette's mother; a membership program was inaugurated; the first catalog of the plant collections was published; and an exhibition of the Arboretum's rare books was held at the Newberry Library in Chicago.

As the last Morton family chairman, Suzette personally selected her successor, Charles C. Haffner III, before retiring in 1977. She returned for the dedication of the new Research Center in 1982, which began a period of expansion in staff and research initiatives within the Research Program.

In 1990, Gerard T. Donnelly, a research scientist with experience in teaching and administration was hired as Executive Director. Donnelly and Trustees launched a series of studies that revealed that in order to retain its reputation, the Arboretum would need to expand and improve its programs, facilities, and grounds. New gardens and visitor facilities were planned, a capital campaign held, and today, the Arboretum enjoys greatly expanded visitor programming and increasing levels of membership.

In 2000, W. Robert Reum became Board Chairman, and with Dr. Gerard T. Donnelly, President and Chief Executive Officer, the Arboretum is recognized around the world for its magnificent collection of plants, and as a leader in research and education.

For an overview of the Arboretum's history, read:
A Great Outdoor Museum: The Story of The Morton Arboretum by James Ballowe (2003). This paperbound booklet traces the history of The Morton Arboretum from 1922 to the present, focusing on how it achieved the vision of its founder, Joy Morton. It includes 36 historic photos and is 28 pages long.