Overcoming initial resistance to his membership of the Labor party (due to a distrust within the party of his father's views as being too radical), Hayden was soon popularly elected as one of the then youngest members of the federal parliament (only 28 years old at the time he entered it). He proved to be a diligent, well-spoken parliamentarian. In 1969, he became a member of the Opposition front bench. When Labor won the 1972 election under Gough Whitlam, Hayden was appointed Minister for Social Security, and in that capacity, among other efforts to promoting reform, introduced the single mothers pension and Medibank, Australia's first system of universal health insurance. On 6 June 1975, he succeeded Jim Cairns as Treasurer, a position he held until the Whitlam government was dismissed by the Governor-General, Sir John Kerr, on 11 November 1975. Labor suffered its worst-ever defeat in the election held a month later, and Hayden was left as the only Labor MP from Queensland.