Her campaign focused on economics. She promised to reverse a planned corporation tax rise and a National Insurance increase. She promised to reduce the size of the state. She cited Margaret Thatcher, Ronald Reagan, and 1980s Chancellor Nigel Lawson as inspirations for her policies. She also promised to help people with the cost-of-living crisis. She won the leadership contest with 57% of the vote.

Truss became Prime Minister on 6 September 2022. She appointed Kwasi Kwarteng as Chancellor, James Cleverly as Foreign Secretary, and Suella Braverman as Home Secretary.

On 8 September, Truss announced the Energy Price Guarantee, which provided a measure of protection against rising household energy costs.

Just after midday, as the measures were debated by the Commons, news arrived that Queen Elizabeth II was seriously ill. She died that afternoon, just after 3pm. That evening, after formal announcement of the news by the Palace, Truss gave a short speech that reflected her thoughts and informed the nation of the regnal name of King Charles III. For the next 10 days, the nation was in official mourning and Truss participated in the events, including attending the Accession Ceremony of Charles III and reading a lesson at the Queen’s funeral service.

Normal politics resumed on Monday 19 September. Truss attended the UN General Assembly in New York on 21 September, meeting with world leaders, including US President Joe Biden.

On 23 September, Chancellor Kwasi Kwarteng announced the mini-budget. The implementation of a free market programme (nicknamed ‘Trussonomics’) was the main policy of the Truss government.  It announced tax cuts, the abolition of the 45p rate on tax, and cancelled the rise in National Insurance.

The budget was extremely poorly received by the financial markets, international bodies, and even some world leaders who were with alarmed at what appeared to be reckless tax cuts and unfunded spending commitments. Both the pound sterling and government bonds fell sharply and interest rates rose. Criticism intensified when it became clear that the financial measures had not been subjected to independent analysis.

Within days, Truss reversed the decisions on tax cuts. Truss sacked Kwarteng on 14 October and appointed Jeremy Hunt as the new chancellor. On 17 October, he spoke in the Commons, with Truss sitting by his side, and dismantled the budget, announcing tax rises and spending cuts.

On 19 October, the Home Secretary, Suella Braverman resigned having admitted sharing an official document via a private email account. Her resignation letter expressed ‘concerns about the direction of the government’. That day, Truss told the Commons ‘I am a fighter, not a quitter’. However, it was becoming clear that support for her from the Conservative backbenchers was beginning to disappear. She met senior Conservatives to shore up her support, but to no avail.

On the afternoon of 20 October, Truss announced her resignation and stated that a leadership election would take place. Within a week, the Conservatives had selected Rishi Sunak to be her replacement.