Ms Mordaunt returns to the MOD after previously working as Minister of State for the Armed Forces from May 2015 until July 2016 and Parliamentary Private Secretary to then-defence secretary Philip Hammond.
She has also previously served as Minister for International Development.
Watch: Penny Mordaunt promoting the role of women in the armed forces in 2016.
The 46-year-old retains the job of minister for women and equalities.
First elected to Parliament as MP for Portsmouth North in 2010, Ms Mordaunt achieved ministerial rank under David Cameron between 2014 and 2016 and was appointed minister for disabled people when Theresa May took power in 2016.
She is possibly best known outside of Westminster for donning her swimsuit in the TV show Splash! in 2014, when she joined celebrities to be trained in diving by Olympian Tom Daley.
The daughter of a paratrooper-turned-teacher, she serves as an acting sub-lieutenant of the Portsmouth-based HMS King Alfred.