Kahn was born in Salt Lake City, Utah to Conrad and Mary Prag, Jewish Polish immigrants who befriended the Mormon leader Brigham Young, and sold supplies during the gold rush. Her family moved to San Francisco, California in 1869. She graduated from the San Francisco Girls' High School in 1883, and received a A.B. from the University of California, Berkeley in 1887. She taught high school English and History. She married Julius Kahn in 1899, who served in Congress until his death on December 18, 1924.
Florence Kahn was elected as a Republican to the 69th Congress, by special election, to fill the vacancy caused by the death of her husband, who was then the Representative-elect. She was reelected to the 70th, 71st, 72nd, 73rd, and 74th Congresses, serving from December 7, 1925 to January 3, 1937. She was the first woman on the House Military Affairs Committee.
Kahn supported Herbert Hoover's unsuccessful campaign against Franklin Delano Roosevelt in the 1932 presidential election. She was an unsuccessful candidate for reelection to the 75th Congress in 1936.
Afterwards, Kahn actively tried to get women involved in politics. She was a member of the American Association of University Women, Hadassah and the Council of Jewish Women. She was a practicing Jew, and belonged to the Emanu-El synagogue in San Francisco.
Kahn died in San Francisco, and was interred in the Home of Peace Cemetery in Colma, California.
The Western Jewish History Center, of the Judah L. Magnes Museum, in Berkeley, California has a large collection of family papers, documents, correspondence, and photographs relating to Florence Prag Kahn and to her husband, Julius Kahn.