In December 2015, he said Mr Trump was "out of his mind" for suggesting a ban on Muslims entering the US.
Mr Johnson also attacked the Republican for his claim that police were afraid to patrol parts of London, fearing attacks by religious extremists.
The former mayor of London said of Mr Trump's hometown: "The only reason I wouldn't go to some parts of New York is the real risk of meeting Donald Trump."
But as we leap forward to November 2016, and Mr Johnson's words of welcome for a "positive" Trump presidency, it's time to take a look back at some of the other occasions Boris Johnson has changed his mind.
After the Turkish President, Recep Tayyip Erdogan, took legal action against German satirist Jan Boehmermann for performing a poem insulting to the Turkish premier, Boris Johnson wrote a poem of his own.
The limerick - the key parts of which we cannot reprint here - won the Spectator magazine's "President Erdogan Offensive Poetry Competition" in May.
More significantly, though, Mr Johnson has several times changed his public stance on whether or not Turkey should join the EU.
During the referendum campaign, he repeatedly warned voters about the prospect of Turkey joining the EU - and the impact migration from the country could have on the UK. That was despite the fact he had previously said keeping Turkey out of the bloc "sent out the worst possible message to moderates in the Islamic world".
Turkish Prime Minister Binali Yildirim told the BBC at the time: "May God help him and reform him."
Since the referendum, however, Mr Johnson has changed his tone again.
In September 2016, he visited Turkey in search of a "jumbo trade deal". During the trip, he called for "a new partnership" between the UK and Turkey, extolling the virtues of his "beautiful, very well functioning" Turkish washing machine.
He also said the UK would "help Turkey in any way" with its bid to join the EU.
The relationship between the outgoing US president and Boris Johnson has had its ups and downs.
In 2008, Mr Johnson supported the then US senator's bid to become the first black American president, saying: "If Barack Obama can do it, it will be the most fantastic boost, I think, for black people everywhere around the world."
This changed when President Obama came out in favour of Britain remaining a member of the EU, and said the UK would be at "the back of the queue" to make trade deals with the US if it voted Leave.
Reacting in an opinion piece for the Sun in April, Mr Johnson said the "part-Kenyan President" suffered from an "ancestral dislike of the British empire" that had led him to remove a bust of Winston Churchill from the Oval Office.
The comments were branded "idiotic" and "deeply offensive" by Churchill's grandson, Sir Nicholas Soames.
In a December 2015 column, Mr Johnson wrote that Russian President Vladimir Putin was a "ruthless and manipulative tyrant" who looked "a bit like Dobby the House Elf" from Harry Potter.
But in August, the foreign secretary called for the UK to "normalise" relations with Russia.
A Foreign Office official said Boris Johnson felt "we need to continue to build a constructive dialogue on issues of mutual concern as well as on points of disagreement".
Mr Johnson also praised the Russian president for assisting in the liberation of Palmyra in Syria, saying Russian troops had driven the so-called Islamic State group from the archaeologically significant site "with a ruthless clarity" that "made the West look ineffective".
As well as his comments about Barack Obama, Mr Johnson once branded Hillary Clinton a "sadistic nurse in a mental hospital".
But when he stood alongside Secretary of State John Kerry at a July 2016 press conference, his US counterpart defended him, saying he had been told the foreign secretary was a "very smart and capable man".
A relieved Mr Johnson said "Phew - let's stop there." But a grinning Mr Kerry nudged his less-experienced colleague, saying: "It's called diplomacy, Boris."
China, 2008: "Ping-pong was invented on the dining tables of England in the 19th Century, and it was called Wiff-waff. And there, I think, you have the difference between us and the rest of the world. Other nations, the French, looked at a dining table and saw an opportunity to have dinner; we looked at it and saw an opportunity to play Wiff-waff. And I say to the Chinese, and to the world, that ping-pong is coming home."
Papua New Guinea, 2006: "For 10 years, we in the Tory party have become used to Papua New Guinea-style orgies of cannibalism and chief-killing, and so it is with a happy amazement that we watch as the madness engulfs the Labour Party."
Uganda, 2002: "If left to their own devices, the natives would rely on nothing but the instant carbohydrate gratification of the plantain."
Foreign Secretary Boris Johnson's praise of Donald Trump's victory in the US presidential election came as a surprise to some, given his previous remarks about him.