Unlike Sam Gyimah – another former leadership candidate in the 21 – Mr Stewart did not turn his back on the Conservatives and join the Liberal Democrats.
He considered standing as an independent in his Penrith and The Border seat but did not want to run against his former Tory colleagues, which would “test loyalties” and “destroy old friendships”.
With Mr Johnson set on an early election, the Tories will hope that a new wave of MPs – replacing some of the pro-EU rebels – share a commitment to Brexit and a looser future relationship with Brussels which could finally end the party’s decades-long schism over Europe.
Mr Stewart’s decision to leave will deprive Parliament – and the Conservatives – of one of the country’s most interesting MPs, with a backstory so colourful there was talk of a Hollywood biopic.
But his political ambitions remain undimmed, with a shot at the London mayoralty in 2020 now his focus.
For someone who was forced to deny claims he was a spy, Mr Stewart put himself firmly in the public eye during the Tory leadership contest with a series of campaign walkabouts, engaging with members of the public in exchanges captured on social media.
Mr Stewart only made it into the Cabinet as International Development Secretary in May, but wasted no time in setting out his stall as a leadership candidate from the moderate, soft-Brexit wing of the party.
The 46-year-old’s past has been so eventful that the rights to one of his books were bought by Brad Pitt’s production company – although Mr Stewart joked that “the story of my life sounded really, really good and was really attractive to Brad Pitt until I became a Conservative MP, at which point he gave up on the whole thing”.
Before becoming an MP in 2010, he had a career including stints in the army and diplomatic service.
He had a short period as an officer in the Black Watch before going to Oxford University, and his diplomatic work saw a posting in Indonesia, a role as British representative to Montenegro in the wake of the Kosovo crisis, and as the coalition deputy-governor of two provinces in southern Iraq following the 2003 invasion.
He spent 21 months walking across Iran, Afghanistan, Pakistan, India and Nepal, staying in 500 village houses on the journey.
One event in his past came to the fore during the Tory leadership campaign – the time he smoked opium at a wedding in Iran.
Mr Stewart claimed the drug “had no effect” on him “because I was walking 25-30 miles a day”.
He said: “I was invited into the house, the opium pipe was passed around at a wedding. I thought – this is going be a very strange afternoon to walk – but it may be that the family was so poor they put very little opium in the pipe.”
Mr Stewart’s past – and the fact his late father was a senior Secret Intelligence Service officer – have led to speculation that he too was in MI6.
It is a claim denied by Mr Stewart, although he admitted he would not be allowed to say he had been a spy if it were true.
On the BBC’s Today programme, presenter Nick Robinson asked: “You can’t really answer the question whether you were a spy or not, you can just simply say you served your country?”
Mr Stewart answered: “I definitely would say I served my country, and if somebody asked me whether I am a spy, I would say no.”