When Sedgwick purchased the Atlantic Monthly in 1908, the monthly circulation was 15,000 and the magazine ran an annual deficit of $5000. He worked quickly to reverse the trend and by 1928, he had increased circulation to 137,000. He has been credited with discovering many writers and with being the first American publisher to print the works of Ernest Hemingway. Sedgwick resigned as editor in 1938 and sold the magazine in 1939.
Sedgwick married Mabel Cabot in 1904. They had four children: Ellery Jr., Cabot, Theodora, and Henrietta. Mabel Sedgwick died in 1937. Sciatica made Sedgwick bedridden for a few months in 1938-1939, and he was also plagued with arthritis. He remarried in 1939 to an Englishwoman, (Isabel) Marjorie Russell. He died in 1960 in Washington, D.C., and is buried in the Sedgwick family plot in Stockbridge.