The visit did not begin well.
Just as the door of 10 Downing Street opened to let him in, John Kerry turned to wave for the cameras.
For the sake of the picture, the policeman inside Number 10 helpfully closed the door. Unfortunately, however, the US Secretary of State didn't realise it had been closed - and so, when he turned to enter, he walked straight into it.
Dear me. We haven't even begun our trade talks with the Americans - and already we've shut the door in their face.
After Kerry had met Prime Minister Theresa May, he went to the Foreign Office to meet Boris Johnson. The two men held a joint press conference.
Now, I know we in the media need to stop referring to the former Mayor of London as "Boris" - he's a politician, not a family pet - but all the same, it did feel incongruous, hearing Kerry soberly refer to him as "Foreign Secretary Johnson".
Boris - sorry, I mean Foreign Secretary Johnson - must have thought so too, to go by the disbelieving grin on his face.
To be fair, though, he did seem to be giving the whole "serious statesman" thing a bash.
He'd even had his hair cut - not smarter necessarily, but at any rate shorter.
Donning a pair of glasses, he began by making a statement on Syria, Yemen and other matters of grave import. As of course it would have to be, the statement was entirely devoid of his usual flair, his usual oomph, his usual... what shall we call it? His usual wiff-waff.
He was doing his best to play the part, but he didn't look at ease.
His problem, I suspect, is that he's just not very good at being boring.
Compare him with his predecessor, Philip Hammond, the new Chancellor of the Exchequer. Hammond has no trouble being boring. He is effortlessly boring. Being boring comes naturally to him. He was born boring.
For Foreign Secretary Johnson, however, being boring is a struggle.
He's desperate to entertain, but knows he mustn't. You can tell it pains him. He wears his solemnity like an itchy vest.
A British journalist wryly asked Kerry if he'd ever met anyone like Foreign Secretary Johnson before. Kerry pretended to have no idea what the journalist was getting at, then said he'd been told that his new friend was "very smart and capable".
"Phew!" said Foreign Secretary Johnson, wiping his brow like a Beano character.
"It's called diplomacy," said Kerry, not very diplomatically.
An American journalist quoted some disobliging remarks that Foreign Secretary Johnson had previously made about US President Barack Obama and Democratic candidate Hillary Clinton.
Foreign Secretary Johnson replied that there was no point apologising for all the disobliging remarks he'd made in the past about foreign dignitaries - because it would simply take too long.
To be fair, I think that's probably true. See, he can be quite practical when he puts his mind to it.