He could be heard quipping “Harry Kane finally scored!” in reference to the Tottenham Hotspur striker’s slow start to the tournament.
But Twitter users were quick to edit the images, mocking up Mr Johnson taking joy in entirely different situations playing out on the screen in front of him.
One showed Boris and his wife Carrie Symonds watching CCTV footage of the former Health Secretary Matt Hancock kissing his close aide Gina Coladangelo in his office in the wake of the scandal.
A second mock-up featured the PM’s former lover Jennifer Arcuri wrapped in a Union Jack.
Another edit showed Mr Johnson celebrating during an episode of the Teletubbies.
Other eagle-eyed users spotted a bin for confidential waste is next to the recycling bin inside No 10.
Among the sea of comments, one person tweeted: “Surely nothing can go wrong here.”
The Prime Minister and his Downing Street staff were just some of more than 20 million people who tuned into the big Euro 2020 clash between old adversaries England and Germany.
Crowds in Trafalgar Square also joined the joyous celebrations of the team’s 2-0 victory in the round of 16.
Now all eyes turn to Rome where England will play Ukraine on Saturday evening.
They hope to secure a place in the semi-final, after Raheem Sterling and captain Harry Kane’s goals sealed Tuesday’s win.