Flinn became politically active in the
Republican Party as a ward boss collecting stray votes. He soon attained office in 1877 as a member of the Board of Fire Commissioners. Flinn quickly partnered with
Christopher Magee, the city's Republican Party political boss. In 1877, he was elected to the
State House to represent the Pittsburgh area by holding one of
Allegheny County's allotted at-large seats. In 1882, Flinn was appointed chairman of the executive committee of the Pittsburgh Republican party, a position he held for the next 20 years, and in 1890 he was elected to the
Pennsylvania State Senate, where he sponsored the Good Roads Act, which became law in 1895. He remained there until his resignation in 1902. From 1884 until 1912 he served as a delegate to every
Republican National Convention. At the May 1912 state convention, Henry G. Wasson, with Flinn's support, was elected chairman of the Republican state committee and Flinn was endorsed for national committeeman, but not formally elected. Their election was seen as a victory for the progressive wing of the party, which essentially took control of the party away from the more conservative
Boies Penrose. Penrose, who had previously served as state committee chairman himself from 1903 through 1905, did not attend the convention, and did not seek re-elected to the post of
Republican National Committeeman. Flinn, however, resigned the chairmanship after less than two months in office. A longtime supporter of
Theodore Roosevelt, he expressed disappointment that the Republican Party chose not to nominate Roosevelt for President in the
1912 election, and instead nominated incumbent
President William Howard Taft. He therefore followed Roosevelt out of the Republican Party, and would later become active in the state's
Progressive Party. Penrose, for his part, would go on to wrest control of the State Party from
Henry Wasson, one of Flinn's lieutenants who was left to lead the party following Flinn's departure. ==Business==