Tory MP Jake Berry told critics of Mr Johnson to stop “sniping from the comfort of Whitehall” and back the Cabinet minister.
He spoke out about the attacks on Mr Johnson, amid concerns among they are damaging the reputation of the Foreign Secretary around the world.
In recent weeks Prime Minister Theresa May and Chancellor Philip Hammond have publicly poked fun at the former London mayor.
Rossendale and Darwen MP Mr Berry’s defence of Mr Johnson came after the Foreign Secretary visited Afghanistan for the first time.
Mr Berry told the Mail on Sunday: “Boris has spent this weekend in Afghanistan, one of the most dangerous countries in the world.
“And what are his armchair critics doing? Sniping from the comfort of Whitehall.
“They need to get behind him if Britain is going to succeed with Brexit.”
His comments echo those of Paul Goodman, the former Tory MP who is now editor of the ConservativeHome website.
He wrote in the Daily Telegraph last week: “Mr Johnson is used to being asked to knock off the jokes. But the same request should now be made of the Prime Minister, at least as far as those at his expense are concerned.”
The Prime Minister used her Conservative Party conference speech to joke “can Boris Johnson stay on message for a full four days? Just about”.
At a recent awards ceremony the Prime Minister also teased him after he had joked about Lord Heseltine “throttling” his dog. Mrs May said: “Boris, the dog was put down ... when its master decided it wasn’t needed any more.”
Mr Hammond made a barbed reference to Mr Johnson’s aborted leadership campaign as he delivered his Autumn Statement in the Commons.
“I suspect that I will prove no more adept at pulling rabbits from hats than my successor as Foreign Secretary has been at retrieving balls from the back of scrums,” Mr Hammond said - a reference to Mr Johnson’s previous comments about his leadership ambitions that “if the ball came loose at the back of the scrum” he would pick it up.
On his visit to Afghanistan, the Foreign Secretary held talks with president Ashraf Ghani and chief executive Abdullah Abdullah in Kabul and praised the work of British military and civilian staff helping to rebuild and stabilise the country.
Mr Johnson said: “Afghanistan is an amazing country and I’m incredibly proud of the work that the UK is doing here to challenge extremism and terrorism, promote democracy and human rights and support the Afghan government’s reform plans.
“British military trainers are improving the ability of the Afghan military to stabilise the country and respond to extremism, and our development works means girls are defying extremists by going to school and university. Our work also means that terrorism is increasingly tackled at source.
“Hundreds of British men and women continue to work here for noble reasons, in often highly dangerous circumstances, and we continue to owe them a huge debt of gratitude for the vital contribution that they are making.”
Mr Johnson also went to the British Cemetery in Kabul to pay tribute at a memorial to the 456 UK servicemen and women who lost have their lives in Afghanistan since 2001.