The Prime Minister had claimed he would not drink until Brexit is sorted – with the first phase of the UK’s withdrawal set for January 31.
But he failed to show restraint and maintain his “do or dry” pledge after pulling a pint in the Lych Gate Tavern on Queen Square.
WATCH: Prime Minister in Wolverhampton on Armistice Day
Mr Johnson, who previously made a “do or die” pledge to deliver Brexit by October 31, was applauded as he entered the pub to speak with veterans after an Armistice Day service at the city’s cenotaph.
The PM held a baby before getting behind the bar to pull a pint of BFG (Bradley’s Finest Golden).
Asked if he would taste the beer, he replied: “I’m not allowed to drink until Brexit is done.”
He added “I’ll whet my whistle” before indulging in a sip.
Pub landlord Dave Buck jokingly held the pint aloft and offered it to customers for £80, but there were no takers.
Mr Buck also joked that Mr Johnson’s pint-pulling technique was “terrible” and said the PM would not be offered a job behind the bar.
But Mr Buck told the PA news agency: “It was a pleasure to meet him and it was in good spirit – pardon the pun – that he came behind the bar.
“Yes, you can tell he’s poured a pint before and possibly drank one.”
Mr Johnson first made reference to his apparent alcohol ban during a hospital visit last Friday.
It emerged less than 24 hours after he was seen sipping whisky during a distillery visit in Scotland on Thursday.