Prime Minister Boris Johnson has replaced Amber Rudd only hours after she stunned Westminster by quitting the Cabinet and the Tory Party.
Therese Coffey MP, an Environment Minister and MP for Suffolk Coastal, has been promoted to Work and Pensions Secretary.
Elected in 2010, Ms Coffey is a former deputy Commons leader and was appointed as Environment Minister by Theresa May.
The Liverpool FC fan backed Remain during the EU referendum in 2016 and voted against legalising gay marriage in 2013.
She holds up former Tory leader Margaret Thatcher as her political hero.
Ms Coffey, who has a doctorate in chemistry, told Conservative Home in 2010: "She stood up for Britain, for enterprise and for freedom. Plus she is the only science graduate ever to have been PM."
Her predecessor Ms Rudd resigned after suggesting the Government was aiming to take the UK out of the EU without a deal.
Speaking on the BBC's Andrew Marr Show, the Hastings & Rye MP said the PM failed to satisfy her that the Government was doing enough to negotiate an agreement with Brussels.
She said: "I have not seen enough work going into actually trying to get a deal.
"When I asked Number 10 for a summary of what the plan was for actually getting a deal, I was sent a one-page summary."
Ms Rudd has already congratulated her successor, tweeting: "Congratulations to my good friend @theresecoffey on her appointment as Secretary of State @DWP. I know she will do an excellent job in a first rate department."
International Trade Secretary Liz Truss called it an "absolutely brilliant appointment".
Ms Coffey, an avid fan of Devon rock band Muse, comes to the Cabinet amid deep political turmoil with Downing Street at loggerheads with Parliament over the future on Brexit.
The PM is in a stand-off with the Opposition and is threatening to disobey a new law demanding that the divorce negotiations are extended until January 2020.
Ms Coffey, having served in the Department for Environment, Food and Rural Affairs (Defra) for the past three years, will be well-versed in Brexit and no-deal planning.
Defra is set to be one of the departments most affected by Britain's divorce from Brussels given its close ties to EU rules on food safety standards, farming and fishing.
Before being promoted, one of her responsibilities was improving air quality after the Government was reprimanded in the courts for failing to tackle rising levels of pollution.
She will now have to get to grips with the controversial Universal Credits rollout, a long-running Government reform which has amalgamated a host of benefits.
There have been reports of recipients being plunged into poverty after being enrolled in the scheme, with critics calling for its roll-out to be paused.
Shadow welfare secretary Margaret Greenwood said that Ms Coffey's "first act" should be to put an end to Universal Credit.
"Therese Coffey is now the fifth Secretary of State for Work and Pensions in the past three years," said the Labour MP.
"At a time when Universal Credit is in crisis, the most vulnerable people in our society need dedication and stability, not more chaos.
"Universal Credit is clearly failing. In her first act as Secretary of State, Therese Coffey must stop the roll-out of Universal Credit as a matter of urgency before any more people are pushed into hardship."