Ms Mordaunt, 44, replaces fellow Brexiteer Priti Patel, who was forced out by Prime Minister Theresa May on Wednesday for not disclosing a series of unauthorised meetings with senior Israelis.
One of her first tasks will be dealing with the growing discontent at Britain’s huge foreign aid spending.
The Daily Express’s Stop The Foreign Aid Madness crusade has attracted an army of supporters, who have completed coupons published in the newspaper urging Mrs May to cut the budget.
We believe the 0.7 per cent budget commitment should be reduced so more money can be spent on the struggling NHS and social care services in Britain.
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Ms Mordaunt, who is probably best-known for donning her swimsuit to take part in the TV show Splash! with Olympian Tom Daley in 2014, dodged questions about whether she was joining a Cabinet “in chaos” last night.
She said: “I’m delighted to have been appointed.
“I’m looking forward to working with the team here to continue building a safer, more secure, more prosperous world for us all and really giving the British public pride in what we do.”
The promotion was widely welcomed by Tory MPs. Meanwhile, charities vowed to work with Ms Mordaunt to champion the cause of spending British aid to help the world’s poorest people.
Many MPs were furious last week when Ms Mordaunt was overlooked in favour of May ally and former chief whip Gavin Williamson to replace Defence Secretary Sir Michael Fallon when he quit over claims about his behaviour.
Former Armed Forces Minister Ms Mordaunt is the daughter of a paratrooper, a Royal Navy reservist and acting sub-lieutenant of HMS King Alfred, which is based in her hometown of Portsmouth.
In 2004, she founded a media company, which she sold in 2010 when she became MP for Portsmouth North at the second attempt.
She has been a government minister since 2014. She has worked as a Tory HQ press offi ce staffer, director of communications at Kensington and Chelsea Council and the Freight Transport Association, and has also been a director of Diabetes UK and the Big Lottery Fund.
But she also has personal experience which is relevant to her new job, having worked in hospitals and orphanages in Romania before and during her time studying philosophy at Reading University.
A former drama student and magician’s assistant, she donated her £10,000 fee from Splash! towards restoring her local lido and to a variety of Armed Forces charities.
She has been replaced as a minister at the Work and Pensions Department by Truro and Falmouth MP Sarah Newton. Her previous job as junior Home Offi ce minister has gone to the Louth and Horncastle MP Victoria Atkins.
Ms Mordaunt’s predecessor Ms Patel yesterday tweeted: “Congratulations to my dear friend Penny Mordaunt – the new International Development Secretary.”