She served as a member of the U.S. House of Representatives from Pennsylvania and was appointed Treasurer of the United States by President John F. Kennedy.

Granahan was a member of the Democratic Party and was the first woman to be elected to the United States Congress from Philadelphia.

[2] She graduated from Easton Area High School and then Mount St. Joseph Collegiate Institute in Philadelphia, which was later renamed Chestnut Hill College.

[3] She served as chair of the House Subcommittee on Postal Operations, and worked with Postmaster General Arthur Summerfield to pass the Granahan bill "to seize and detain the mail of anyone suspected of trafficking in obscenity.

In 1965, Granahan suffered a blood clot after a fall, and worked a reduced schedule following the incident.