After Holt's death in 1967, McMahon wished to contest the Liberal leadership but had his candidacy vetoed by John McEwen, the leader of the Country Party.

McMahon initially continued on as Treasurer in the Gorton government, but in 1969 was demoted to Minister for External Affairs after an unsuccessful challenge for the leadership.

Gough Whitlam's Labor Party defeated McMahon at the 1972 federal election, ending 23 consecutive years of Coalition rule.

McMahon has been described as one of Australia's worst prime ministers by Australian political scientists and historians,[1][2][3][4] and after leaving office several of his former colleagues openly criticised his leadership style and personal character.

Whitlam, his successor, acknowledged him as "an extraordinarily skilful, resourceful and tenacious politician", and credited him with having prevented a larger margin of defeat in 1972.

He was the third of five children born to solicitor William Daniel McMahon and Mary (née Walder), daughter of a sailmaker; an older brother predeceased him.

[6] He moved home frequently as he was shifted between family members, living for periods in Kensington, Beecroft, Gordon, and Centennial Park.

He won his university's lightweight boxing title, and in his final year at Sydney Grammar rowed in the Head of the River race.

However, he managed to preserve the influence and independence of his department, and in fact made a number of cabinet submissions that were contrary to McEwen's wishes.

McMahon oversaw the creation and administration of what became the National Service Act 1964, which re-introduced compulsory conscription for 20-year-old males in anticipation of further Australian involvement in South-East Asia.

On the labour side of his portfolio, he frequently came into conflict with the leadership of the Australian Council of Trade Unions (ACTU), though there was no major industrial action during his tenure.

He frequently found himself on the defensive as Whitlam attacked the increasingly unpopular Vietnam War and advocated radical new policies such as universal health insurance.

In June 1971, McMahon cancelled Gorton's planned nuclear power program, which had included a reactor capable of generating weapons-grade plutonium.

Late on election night, with the result beyond doubt, McMahon conceded defeat, ending the longest unbroken run in government in Australian history.

[27] However, he believed that Governor-General John Kerr had acted unconstitutionally in dismissing the prime minister, and said that he would have challenged the decision in the High Court if he had been in Whitlam's position.

[28] McMahon believed that those responsible for the "loans affair" – including Whitlam and several of his ministers – had acted illegally and should be prosecuted for their involvement.

[29] Prior to the 1977 election, McMahon was unsuccessfully challenged for Liberal preselection by John Abel, whose Division of Evans had been abolished in an electoral redistribution.

McMahon left parliament in January 1982, citing dissatisfaction with the 1981 budget as a major factor in his decision to retire before a general election.

[32] He nominated future prime minister Malcolm Turnbull as his preferred successor in Lowe,[33] but the Liberal Party chose another candidate Philip Taylor.

They were rejected by six publishers, and reviewers (who included Barry Jones and Phillip Adams) considered them to be poorly written and overly detailed.

[36] Later that year, he described Andrew Peacock's hold on the Liberal leadership as "very, very fragile", and tacitly endorsed John Howard as a future leader.

The wedding was held three months later at St Mark's Church, Darling Point, followed by a reception for 400 people at the Royal Sydney Golf Club.

[43][44][45][46] The suggestion was repeatedly denied by Lady McMahon;[41] one occasion in the 1970s resulted in an infamous tabloid headline "My Billy's No Poofter – Sonia Tells".

In 1953, he gave an address to the Australian Institute of Political Science in which he explained how he believed Christian doctrines necessitated parliamentary democracy and a market economy.

[52] Similarly, The Age surveyed eight historians in 2004 and all but one ranked McMahon as Australia's worst prime minister since World War II.

Donald Horne called him "perhaps the silliest prime minister we ever had",[55] and Peter Ryan said that "McMahon's way of politics was one of lying and leaking, conniving and conspiring, deceit and double-crossing".

[59] Laurie Oakes, who spent more than 50 years in the Canberra Press Gallery, viewed McMahon as "a liar and a sneak" and rated him as the worst prime minister he had worked with.

[59] Mungo MacCallum, while noting that he left no lasting achievements, called his prime ministership a "brief but cheerful interlude" and praised him for leaving office with good grace.

[62] Marian Simms compared McMahon to Richard Nixon, suggesting that his character traits have been overemphasised,[53] while Troy Bramston viewed him as "a prime minister who clearly understood the challenge of the times and was fighting to get his ship back on course" when he was forced out of office.

[63] In his memoirs, Gough Whitlam wrote that McMahon was "an extraordinarily skilful, resourceful and tenacious politician ... had he been otherwise, the ALP victory in December 1972 would have been more convincing than it was".