Born in an Ozark Mountain log cabin in Harrison, Arkansas, Pfost was five when her parents moved to a farm near Boise, Idaho, in 1911. One of five siblings, she quit Meridian High School at 16 in 1922 and worked as a milk analyst at a dairy in Nampa. The next year she married her supervisor, Jack Pfost, who was more than twice her age. She later graduated from Link's Business College in Boise in 1929. Pfost entered politics in Canyon County; she held several positions in county government between 1929 and 1951, including deputy county clerk, auditor, recorder of deeds, and county treasurer. She also served as an Idaho delegate to all Democratic National Conventions between 1944 and 1960. The Pfosts ran a real estate business in the 1940s and into the 1950s.
Though her House seat was considered secure, the death of Henry Dworshak in July 1962 prompted Pfost to run for his seat in the U.S. Senate. She was the Democratic nominee in the special election, but was narrowly defeated by the appointed Republican incumbent, former Governor Len Jordan. The election took place shortly after the Cuban Missile Crisis of late October; Jordan was re-elected in 1966 and retired at the end of that term, in early 1973. Pfost's congressional seat was won by six points by Democrat Compton White, Jr. of Clark Fork, the 31-year-old namesake son of the late eight-term congressman. Idaho's other House seat also went to young Democrat, as 33-year-old Ralph Harding of Blackfoot won a second term. Idaho's other U.S. Senate seat was also on the ballot, with 38-year-old Democrat Frank Church of Boise re-elected to the second of four terms. Through 2019, Dworshak's seat, earlier held by William Borah for over three decades, has been continuously held by Republicans for over seventy years, and Idaho has yet to elect a woman to the U.S. Senate.
After leaving the House in 1963, Pfost remained in Washington and worked in the Federal Housing Administration as a special assistant on housing for the elderly. She was hospitalized in Washington with pneumonia in October and a few months later at Johns Hopkins Hospital in Baltimore, Maryland. Later diagnosed with Hodgkin's disease, Pfost was admitted to Johns Hopkins Hospital several times in 1965, and died there on August 11 at age 59. Pfost's husband Jack died of a heart attack four years earlier, at her Washington office during her last term in Congress. They did not have children and are buried at Meridian Cemetery in Meridian, Idaho, near her parents, William L. and Lily E. Wood Bowers, and other family members.
