In 1944, it was the site of the Dumbarton Oaks Conference, which developed plans for the founding of the United Nations following World War II.

The Blisses engaged the architect Frederick H. Brooke (1876–1960) to renovate and enlarge the house (1921–1923), thereby creating a Colonial Revival residence from the existing Linthicum-era Italianate structure.

In 1938 they engaged the architect Thomas Tileston Waterman (1900–1951) to build two pavilions to house their Byzantine Collection and an 8,000-volume library, and in 1940 gave Dumbarton Oaks (which included about 16 acres (65,000 m2) of land) to Harvard University, Robert Bliss's alma mater.

Wishing to increase the scholarly mission of Dumbarton Oaks, in the early 1960s the Blisses sponsored the construction of two new wings, one designed by Philip Johnson (1906–2005) to house the Robert Woods Bliss Collection of Pre-Columbian Art and its research library and, the other, a garden library designed by Frederic Rhinelander King (1887–1972), of the New York City architectural firm Wyeth and King, to house the botanical and garden architecture rare books and garden history reference materials that Mildred Bliss had collected.

Igor Stravinsky conducted the concerto in the Dumbarton Oaks music room on April 25, 1947 and again for the Bliss's golden wedding anniversary, on May 8, 1958.

He also conducted the first performance of his Septet, which is dedicated to the Dumbarton Oaks Research Library and Collection, in the music room on January 24, 1954.

[9] In the late summer and early fall of 1944, at the height of the Second World War, a series of important diplomatic meetings took place at Dumbarton Oaks, officially known as the Washington Conversations on International Peace and Security Organization.

During the 1970s it funded major fieldwork projects work in Cyprus, Syria, and Turkey, efforts that today span the entire geographical breadth of the former Byzantine commonwealth.

In 2008 the institute also completed an extensive renovation of the main house and museum wing, including restoration of its historic period rooms, several of which were created by the Parisian designer, Armand-Albert Rateau (1882–1938).

The mission of Dumbarton Oaks is to support and promote scholarship in three areas of study: Byzantine, Pre-Columbian, and garden and landscape architecture.

The program focuses on the cultures that thrived in the western hemisphere from northern Mexico to southern South America, from the earliest times to the sixteenth century.

Mildred and Robert Woods Bliss initiated these collections in the first half of the twentieth century and provided the vision for future acquisitions even after giving Dumbarton Oaks to Harvard University.

In addition to its Byzantine holdings, the collection includes Greek, Roman, and western medieval artworks and objects from the ancient Near East, pharaonic and Ptolemaic Egypt, and various Islamic cultures.

The Robert Woods Bliss Collection of Pre-Columbian Art comprises objects from the ancient cultures of Mesoamerica, the Intermediate Area, and the Andes.

Among its most important holdings are a variety of sculptures in stone, including carvings of Aztec deities and animals and several large relief panels bearing the likenesses of Maya kings.

Gold and silver objects from the Chavín, Lambayeque, Chimú, and Inca cultures offer evidence of the expertise achieved by Andean metalsmiths, and over forty textiles and works in feathers testify to the importance of fiber arts in this region.

The ceiling and flooring of this room were inspired by examples at the guardroom of the historic Château de Cheverny near Paris and were fabricated by the Parisian designer, Armand Albert Rateau.

This building—eight domed circular galleries (having an unroofed fountain area at the center) set within a perfect square—recalls Islamic architectural ideas, and Johnson later credited the design to his interest in the early sixteenth-century Turkish architect Mimar Sinan.

Johnson maintained that he wanted the garden to "march right up to the museum displays and become part of them,"[16] with the plantings brushing the glass walls and the sound of splashing water audible in the central fountain.

Each interconnected exhibition gallery is twenty-five feet in diameter, having curved glass walls supported by cylindrical columns sheathed in Illinois Agatan marble and shallow domes that rise from flat bronze rings.

In 1964, the Research Library acquired Robert Woods Bliss's personal collection of 2,000 rare and important works on Pre-Columbian art history, anthropology, and archaeology, which has since grown to more than 32,000 volumes, and Mildred Bliss's garden library, including rare volumes and prints, which now includes 27,000 books and pamphlets.

Her library was enlarged, with advice from Beatrix Farrand, designer of the Dumbarton Oaks garden, once Mrs. Bliss conceived the idea in the 1950s of starting a program of studies in landscape architecture.

The herbals represent early attempts to create a coherent system of plant description and are forerunners of today's science of taxonomy.

These techniques were also used by artists who created the newly popular still lifes of flowers and fruits, and by artisans, such as jewelers, tapestry weavers, and furniture decorators, in pattern books recording their floral designs.

In addition to printed books, there is a collection of manuscripts and drawings covering the same range of topics – a late 17th-century planting plan for an Italian garden; Hans Puechfeldner's fine images of late 17th-century mannerist gardens; a number of paintings by oriental artists executed for western patrons to record discoveries of new plants made during the expansion of Europe into the East.

The Library owns the original watercolors for Buchoz's Collection des Fleurs dans les Jardins de la Chine as well as the gouaches by Clara Maria Pope for Samuel Curtis's Beauties of Flora.

Noteworthy acquisitions from recent years are Francesco Colonna's La Hypnerotomachia di Poliphilo, 1545 edition; Salomon de Caus's La pratique et demonstration des horloges solaire, published in Paris in 1624; and Humphry Repton's album of 500 engraved views taken from William Peacock's Polite Repository, arranged by year.