Ian McCahon Sinclair AC (born 10 June 1929) is an Australian former politician who served as a Member of Parliament for 35 years, and was leader of the National Party from 1984 to 1989.

He spent his final six months in parliament as Speaker of the House of Representatives, following the sudden resignation of Bob Halverson; he is the only member of his party to have held the position.

[3] His father was a chartered accountant who also served as deputy mayor of Ku-ring-gai Council, chairman of Knox Grammar School, and an elder of the Presbyterian Church.

[10] When Country Party leader John McEwen retired in February 1971, Anthony was elected as his replacement and Sinclair defeated Peter Nixon for the deputy leadership.

[14] In 1973, Sinclair was one of the six Country MPs to vote in favour of John Gorton's motion calling for the decriminalisation of homosexuality.

[3][16] In 1978, New South Wales Attorney-General Frank Walker appointed Michael Finnane to inquire into the financial dealings of Sinclair's father George, who had died in January 1976.

[19] His supporters criticised the report on several grounds, including that the inquiry was conducted in secret, that its release prejudiced Sinclair's right to a fair trial, and that it was politically biased as both Walker and Finnane were members of the ALP.

[20] In April 1980, Sinclair was charged with nine counts of fraud, relating to forging, uttering, and making false statements on company returns.

Sinclair was elected as his replacement on 17 January 1984, defeating Stephen Lusher by an unspecified margin (the party did not release the results of leadership ballots).

[23] In an interview with Australian Playboy in July 1984, Sinclair acknowledged a previous extramarital relationship with socialite Glen-Marie North.

In the lead-up to the election, Sinclair controversially attributed the spread of HIV/AIDS in Australia to the Labor Party's recognition of de facto relationships and normalisation of homosexuality.

The proposal, also supported by shadow foreign minister Andrew Peacock, was designed to "minimise the harmful policies of major protectionist trading nations" like the U.S. and the European Communities.

[32][33] He opposed trade sanctions on Fiji following the 1987 coups d'état and was accused by foreign minister Bill Hayden of sympathising with the perpetrators.

[39] The following month, following pressure from Howard, he sacked National Senate leader John Stone from the shadow ministry for making similar comments, "with regret".

[41] In May 1989, there were simultaneous leadership challenges in both Coalition parties, with Peacock displacing Howard as Liberal leader and Charles Blunt replacing Sinclair.

The immediate trigger for Sinclair's defeat was dissatisfaction with his conditional support for the Hawke government's deregulation of the wheat industry.

[42] When Blunt lost his seat at the 1990 election, Sinclair made an attempt to regain the NPA leadership, but was defeated by Tim Fischer, and retired to the back bench.

[43] In March 1993, aged 63, ten days after the Coalition lost the 1993 federal election, Sinclair unsuccessfully challenged Tim Fischer for the party leadership.

[44] By 1993, Sinclair was the Father of the House, the only sitting MP to have served with Robert Menzies, and "the last of the Right Honourables" (MPs with membership in the Privy Council).

When Speaker Bob Halverson suddenly resigned in March, Sinclair was elected to replace him, the first National to hold the post.

[47] On 2 April, he named Crean apparently for shaking his head, although Sinclair maintained it was due to "disorderly conduct".

[53] Sinclair also served for several years as the Honorary President of the Scout Association of Australia, New South Wales Branch, retiring in 2019.