 Stephenson joined the Liberal Party and was elected to Sheffield City Council, becoming Lord Mayor in 1908–09 and again in 1910–11.In 1910, he also became the Pro-Chancellor of the University of Sheffield, succeeding Sir Frederick Mappin.At the 1918 general election, he became the first Member of Parliament for the Sheffield Park constituency, holding the seat as a National Liberal in 1922, but dropping to a distant third place in 1923.