The 46-year-old's appointment this week continues a rapid rise through the ranks that has fuelled speculation that she might run for Prime Minister Theresa May's job when the time comes.
For now, however, she says she is "delighted" to take a role for which she has rare experience and which she is said to have been long angling for.
Mordaunt was promoted from international development secretary on Wednesday after Gavin Williamson was summarily sacked as defence secretary following an inquiry into a leak from the government's National Security Council.
She took her place alongside military chiefs and Prince William at a ceremony at Westminster Abbey in London on Friday marking 50 years of Britain's at-sea nuclear deterrent.
Unlike many cabinet ministers, Mordaunt has experience in her brief: she is a Royal Navy reservist and has served as a junior defence minister.
Her father was a paratrooper, who reportedly named her after a World War II cruiser, HMS Penelope, and she also represents the naval city of Portsmouth.
Most voters know Mordaunt for her strong support for Brexit in the 2016 referendum campaign.
However, she drew criticism for repeatedly saying Turkey was about to join the EU and that the UK would not be able to veto the move -- a claim dismissed as "completely wrong" by the then prime minister David Cameron and EU leaders.
Her continued enthusiasm for leaving the bloc has prompted reports she could quit the government over May's handling of Brexit, but has made her popular with grassroots members of the Conservative Party.