After Brandon Lewis resigned, Shailesh Vara took over as Northern Ireland secretary.
Mr Lewis was one of the previously-loyal cabinet members who told the prime Minister on Wednesday that he had to resign due to a loss of support.
Johnson resigned earlier.
On Thursday, Mr Vara began to speak with political leaders in Northern Ireland by phone.
Michelle O'Neill, Sinn Fein's first Minister Designate, tweeted that she had told Mr Vara that he was here for only a short time and that he must respect the outcome of the assembly election and work constructively to get Stormont moving, so a local executive can establish a budget and assist workers and their families.
In time for the Tory party conference, October 10, a new prime minister will be in office.
He said that a Conservative leadership contest will be held this summer. In the meantime, Johnson will remain prime minister.
Before becoming a minister in Northern Ireland Office, Vara served for five months. He then quit in protest against the deal that Theresa May proposed with the EU in November 2018.
He called his return to Northern Ireland Office a "huge honor"
Brandon Lewis was elected Secretary of Northern Ireland in February 2020.
His handling of Stormont's power-sharing crisis and the Northern Ireland Protocol had drawn criticism from political parties.
On Thursday, Mr Lewis tweeted that a "decent" and responsible government is one that relies on honesty, integrity, and mutual respect.
He stated that it was a matter for deep personal regret that he had to "leave" government because he didn't believe these values were being upheld.
Shailesh Vara, a minister from Northern Ireland Office in 2018, was not well-known until he fought against Theresa May’s Brexit deal.
After only five months, he quit the job because the deal proposed would put the UK in a "halfway home" with the EU. However, the UK is still adhering to its custom rules.
He warned that Northern Ireland would have a "different relationship to the EU than the rest of the UK", and added that the "constitutional integrity" of the UK must also be respected.
However, Mr Vara voted later for the Theresa May suggestions - even if reluctantly.
Nearly four years later, he will be the man who steers the controversial protocol bill of the government through the House of Commons in the next fortnight.
If he is appointed secretary of state, he will ensure that it passes into law.
His appointment as a hardline Brexiteer will please the Democratic Unionist Party, DUP and other unionists who want to see the protocol change.
Much will depend on his new boss at Number 10.
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In a letter to the prime minster, Mr Lewis stated that a decision to quit government should not be taken lightly at this critical time in Northern Ireland's history.
He claimed he had "served loyally" in the defense of the government, "both publicly as privately".
"We are, however, now beyond the point of no returns.
"I can't sacrifice my personal integrity in order to defend them as they are now.
"It is evident that our party, parliamentary co-workers, volunteers, and the country deserve better."
On Thursday, Mr Lewis was the first cabinet minister who resigned.
In a shocking move, the prime minister fired Levelling Up Secretary Michael Gove on Wednesday.
After more than 40 ministers and their aides had resigned, his sacking was a record for a 24-hour period.
On Thursday morning, the Conservative Party and government resignations surpassed 50.
Conor Burns, Northern Ireland Office minister, tweeted Thursday that he would be leading for the government at Friday's meeting of the British Irish Council.
He stated that it was crucial that the work at the Northern Ireland Office continued to be done.
Brandon Lewis' memorable quote was made while he was standing in the House of Commons in 2020.
He spoke to MPs about his government's plans to violate international law by amending the Brexit agreement with the EU. He said that the plan was to do this in a "very limited and specific way".
In the same way, his legacy in Northern Ireland might be remembered.
He was only able to work for two-and-a half years and spent most of his time dealing with the pandemic.
There were also standout moments that will be cherished, and some of these will go down in history.
The Northern Ireland Protocol legislation is one of the key issues that will fill the in-tray for the new secretary to Northern Ireland. This controversial bill, which aims at ending legacy prosecutions of Troubles victims and restoring devolution at Stormont, is one of the most important.
Over its unilateral override of the Northern Ireland Protocol, the UK government has been subject to strong EU criticism.
This protocol is part the Brexit deal, which was signed by the prime minister in 2019, and later ratified at Parliament.
The government claims that it is now causing too much political and practical problems.
The EU claims that the UK's attempt to alter the agreement is illegal.
Last month, Parliament cleared the first hurdle for government plans to override certain parts of the Brexit deal that relates to Northern Ireland.
The House of Commons passed controversial legislation to end Troubles legacy prosecutions despite continued political opposition.
The bill provides a conditional amnesty for those accused of killings or other Troubles-related offenses.
The House of Lords will then examine the matter further.
The bill has been opposed by all Northern Ireland MPs. It has also been criticised and criticized by victims' groups as well as relatives.
Another important issue facing the secretary of state is the restoration and maintenance of Northern Ireland's political institutions.
Sinn Fein won the most seats at the May assembly election. However, the Democratic Unionist Party, (DUP) came in second. The DUP is refusing to reenter a power-sharing executive unless the party's concerns regarding the Northern Ireland Protocol are addressed.