Khan was voted into the role last week and his picture now appears on the @MayorofLondon account.
Unfortunately, this means the Labour Mayor's face also appears next to all of Johnson's old tweets.
The Mayor's account biography now has the message: "Tweets before 9 May 2016 are from the previous Mayor".
But some thought this was not enough to resolve the confusion, which makes it appear that Khan has tweeted some sentiments the pro-EU politician certainly wouldn't agree with.
And supporting Tory chancellor George Osborne's budget:
Aside from the amusment, some commenters also appeared to take issue with Khan's lack of neckwear on his first day as Mayor:
While one seemed to have missed the fact that elections happened at all: