Post was born as Emily Price in Baltimore, Maryland, and was born into privilege as the only daughter of famous architect Bruce Price and his wife Josephine Lee Price. She was educated at home and attended Miss Graham's finishing school in New York where her family had moved. A popular debutante, she married society banker Edwin Main Post in 1892 and had two sons, Edwin M. Jr. (1893) and Bruce Price (1895). The couple divorced in 1905, due to her husband's infidelity.

At the turn of the century financial circumstances had compelled her to begin to write, and she produced newspaper articles on architecture and interior decoration, stories and serials for such magazines as Harper's, Scribner's, and the Century, as well as light novels, including Flight of the Moth (1904), Purple and Fine Linen (1906), Woven in the Tapestry (1908), The Title Market (1909), and The Eagle's Feather (1910).

She wrote in various styles including humorous travel books early in her career. In 1922 her book Etiquette was a best seller, and updated versions continued to be popular for decades.

After 1931 Post spoke on radio programs and wrote a column on good taste for the Bell Syndicate; it appeared daily in some 200 newspapers after 1932.

In 1946, she founded The Emily Post Institute which continues her work. On September 25, 1960 she died in her New York City apartment at the age of 86.

Peggy Post, Emily's great-granddaughter-in-law is the current spokesperson for The Emily Post Institute — and writes etiquette advice for Good Housekeeping magazine.