Dominic Raab was the only contender in the race to lead the Conservative Party whose Pro-Brexit views were stronger than those of the new prime minister.
Mr Johnson has now sought to keep him onside with promotion to foreign minister.
Although he has little Middle East experience, Mr Raab spent six years as a government lawyer in the Foreign Office when the legal position on preparations for the Iraq war was the hottest issue in politics.
He has defended his role by saying he worked under Elizabeth Wilmshurst, a senior lawyer who resigned after warning that the war plan was illegal.
In the previous government of Theresa May, he spent four months as the minister in charge of Brexit.
While there, he managed to confuse the Red Sea – which runs from Egypt’s Suez down to Bab Al Mandeb off Yemen – with the Irish Sea when talking about trade barriers with Europe in a parliamentary session.
After the staunch eurosceptic came last in the second round of the leadership race, he gave his backing to Mr Johnson, which led him to become the UK’s senior diplomat.
Jeremy Hunt, Mr Johnson’s Conservative Party leadership rival, resigned as foreign secretary in the middle of a worsening standoff with Iran over captured tankers.
Mr Hunt won about a third of votes from Conservative Party members, but refused to become defence minister, a demotion from his post as foreign secretary.
Mr Raab's experience as the Brexit Secretary gave him a background of dealing and negotiating with senior international figures.
He quit the job last November over the withdrawal agreement with the EU because he said it would leave the UK too closely aligned to the bloc.
In his resignation letter, he slammed the “indefinite” Northern Irish backstop, an insurance arrangement to stop a hard border on the island of Ireland that is hated by pro-Brexit figures.
During a televised debate in the early stages of the Conservative Party leadership race, Mr Raab was keen to burnish his credentials as the man who could stand up to Brussels and renegotiate the agreement.
“They said I pushed them too hard, they said I told them things that no one else had ever dared," he said.
"Well about time too. The truth is I just made clear we couldn’t accept that backstop and that our future relationship must be based on a best-in-class free trade agreement, not the cage of a Customs union.
“I offered constructive solutions, I predicted the deal would be rejected if we didn’t secure them and ultimately, I resigned because I wasn’t in good conscience willing to walk a bad deal over the line."
He courted controversy as the first top-rank politician to argue that suspending Parliament could force a no-deal Brexit through, even though most MPs are strongly against a clean break from the EU.
Mr Raab other Foreign Office experience is working as a lawyer and adviser on the Arab-Israeli conflict, EU law and Gibraltar.
He reflected on his time in the Middle East in a 2010 blog, in the same year he was elected to Parliament, despairing over the state of the peace process.
“Israel must find a way to extricate herself from a conflict that saps her strength, and compounds her isolation,” he wrote.
Mr Raab also worked at The Hague, focusing on bringing in suspected war criminals including Slobodan Milosevic, Radovan Karadzic and Charles Taylor.
But diplomats may be concerned about Mr Raab criticising the former UK ambassador to the US, Sir Kim Darroch, after the leak of memos in which he called the Trump administration inept.
Mr Raab said the comments were too personal.
On Tuesday, he said it was important to have a strong relationship with the US and Mr Trump.
“I’m proud that we’ve got an American president who, when he last came here, had a lot of warm things to say about this country,” Mr Raab told Channel 4.
“He’s been a contentious figure but if you want strong relationships between the people of this country and the people of the United States, if we want to get from the back of the queue to the front of the queue for a trade deal, I think it’s good to have warm relations.”