She was supervisor of public assistance in the State Auditor General's Department, and liaison officer between that department and Pennsylvania Department of Public Assistance, from 1940 to 1943.
Her husband, Rep. William T. Granahan served in Congress from 1945 to 1947 and from 1949 until his death in 1956. His death came shortly after the 1956 primary election and the Democratic Party in Philadelphia selected Kathryn to succeed her husband in both the special election and general election. She was simultaneously elected to serve out her husband's term in the 84th United States Congress and a term in the 85th United States Congress in her own right.[3]
She served as chair of the House Subcommittee on Postal Operations, and worked with Postmaster GeneralArthur Summerfield to pass the Granahan bill "to seize and detain the mail of anyone suspected of trafficking in obscenity."[4]
Granahan began her term as Treasurer on January 9, 1963 after her term in Congress ended. In 1965, Granahan suffered a blood clot after a fall, and worked a reduced schedule following the incident. In 1966, an effort to declare her incompetent failed; however, she resigned four months later.[3]