The stretch of Massachusetts Avenue between Scott Circle and Sheridan Circle, about a mile and a quarter, was the Millionaires' Row of Washington in the decades around 1900. New money from Pittsburgh, Chicago, the West, and the Gilded Age industries arrived in the capital, hired the best architects, and built. Stanford White designed the Patterson House at 15 Dupont Circle. Carrere and Hastings designed the Townsend House, now the Cosmos Club. Jules Henri de Sibour, a French-born Beaux-Arts specialist, designed seven separate buildings along this stretch, including the Clarence Moore House (now the Embassy of Uzbekistan), the Ingalls House, the McCormick Apartments, and the Alexander Stewart House (now the Embassy of Luxembourg). These were houses meant to project status in a city that had relatively few opportunities to do so. The income tax, the Depression, and the suburbanization of American wealth between 1913 and 1945 made most of them economically impossible to maintain as private residences.

Between the wars, foreign governments started buying. An empty mansion with grand reception rooms, conveniently located on a major avenue close to the State Department and the White House, was exactly what an embassy needed. The British had built their own purpose-designed embassy on Massachusetts Avenue earlier than most, but the postwar boom in diplomatic missions in Washington meant that nearly every available mansion eventually found a buyer. The Walsh-McLean House at 2020 Massachusetts (Embassy of Indonesia), the Joseph Beale House at 2012 Massachusetts (Embassy of Portugal), the Halliday House at 2234 Massachusetts (Embassy of Ireland), the Brodhead-Bell-Morton Mansion at 1500 Rhode Island Avenue (Embassy of Hungary), the T. Morris Murray House at 2107 Massachusetts (Embassy of India). The mansions that had been built to show off old families were repurposed to show off newer countries.

The change of use did not always go smoothly. In the early 1970s a developer announced plans to tear down the historic townhouses at 1722 through 1728 Massachusetts Avenue and replace them with an office tower. The fight that followed produced the Massachusetts Avenue Historic District in 1974, which formally protected most of the corridor between 17th Street and Observatory Circle from further demolition. The district preserved not only the mansions still in use as embassies but also the surviving rowhouses, apartment buildings, and commercial structures from the era. Other landmarks along the row include the Phillips Collection at 1600 21st Street (founded by Duncan Phillips in 1921 as the first museum of modern art in America), the Cosmos Club at 2121 Massachusetts, and the Sulgrave Club at 1801 Massachusetts.

The northern end of Embassy Row culminates at Observatory Circle, where the United States Naval Observatory sits on seventy-two wooded acres. The Vice President's official residence, Number One Observatory Circle, is here. Built in 1893 as the home of the superintendent of the Observatory, the house was the official residence of the Chief of Naval Operations from 1923 to 1974, when Congress designated it the Vice President's residence. Walter Mondale was the first vice president to live there full-time. Nelson Rockefeller had been offered it and refused, finding it too modest. Embassy Row's western end also includes the South African embassy at 3051 Massachusetts (the site of a famous 1984 anti-apartheid protest by Randall Robinson and others that helped trigger the Comprehensive Anti-Apartheid Act), the Embassy of the Vatican (the Apostolic Nunciature at 3339 Massachusetts), and the British Embassy at 3100 Massachusetts. The Edwin Lutyens-designed British embassy, opened in 1930, is one of the few purpose-built embassies on the row and remains one of the largest.

Most days the embassies are closed to the public. Twice a year, that changes. Passport DC, an initiative launched by the European Union embassies in 2007 and extended to other countries in 2008, opens the doors of dozens of embassies on a single weekend each May. Lines stretch around blocks. People who have lived in Washington for decades use the chance to walk through buildings they had only ever passed on the sidewalk. The Embassy Series, organized separately since 1994, hosts classical music concerts in embassy reception rooms throughout the year. Three blocks south, between Scott and Dupont Circles, the buildings that were once mansions have been repurposed again, this time as the headquarters of major Washington think tanks: Brookings at 1775, Carnegie Endowment at 1779, the American Enterprise Institute in the old McCormick Apartments at 1789. Some Washingtonians have started calling that segment Think Tank Row. The buildings remember being mansions. The mansions remember being houses. The houses remember the people who built them.