Mr Williamson was sacked by Prime Minister Theresa May after allegedly leaking plans that would see Chinese telecoms company, Huawei, help to build the UK’s 5G network.
He has denied the claims but Downing Street said it had “compelling evidence” and considers the matter now closed.
Ms Mordaunt becomes the first female to take up the role as she leaves her position as International Development Secretary, which she had held since November 2017. Her other previous posts include Minister of State for Department for Work and Pensions.
She has been the serving MP for Portsmouth North since 2010 and comes from a family with a strong history in the armed forces, with her father serving in the parachute regiment.
Ms Mordaunt is a Royal Navy reservist and becomes the first serving reservist to serve as Defence Secretary.
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