Details of the intervention were revealed on Twitter by Grimshaw partner Mark Middleton, who said Johnson called the design "too boring".
"My Boris Johnson story," tweeted Middleton, amid the media storm over news that police had been called to Johnson's home after a loud row with his partner Carrie Symonds.
"Whilst Mayor of London he criticised the proposed elevations of London Bridge Station as too boring," Middelton wrote in the tweet. "We presented as he sat slumped in a seat with his feet on the table & suggested we embellish them with a row of gargoyles"
Johnson, now frontrunner is the race to become UK prime minister, met partners at Grimshaw several times during the process of re-designing RIBA award-winning London Bridge Station, a major infrastructure project in central London completed last year.
During these meetings, the then-mayor called the elevation plans "boring", and suggested that the brickwork arches should be covered with gargoyles to make it more interesting.
This was delivered, Middleton told Dezeen, while Johnson had his feet on the desk of the presentation room.
Johnson is currently one of two candidates in the running to become the leader of the UK's Conservative Party and therefore the prime minister.
"It's just a bit boring, isn't it"
The two meetings Middleton had with Johnson and the Greater London Authority (GLA) were more of a "courtesy" he said, as the designs had already been given planning permission at this stage.
"At the first meeting he kept asking why it wasn't like [London terminus] St Pancras, why didn't it have a big roof?" said Middleton.
"We didn't get into the aesthetics of it. He couldn't accept that the space underneath was a grand space," he added. "He kept giving me a lesson on [Victorian architect] Gilbert Scott."
George Gilbert Scott, was a Gothic revivalist architect designed the ornate 1873 Midland Grand Hotel that is the frontispiece to William Henry Barlow's 1868 St Pancras Station.
At the second meeting, where Grimshaw Architects presented the elevations, Johnson responded negatively to the design.
"He walked into the room and put his feet up on the table, leant back in his seat with his arms folded," said Middleton.
After the presentation, Middleton and his co-presenter invited questions.
"And he says, well, it's just a bit boring, isn't it? It's the facade, the brick, it's just a bit boring. It needs something else. It needs gargoyles along the front."
"He was chuckling to himself"
The exchange continued with Middleton asking Johnson to clarify his position on the gargoyles.
"He was chuckling to himself, and all his colleagues were chuckling away to themselves," Middleton said.
This behaviour was very unusual, Middleton said.
"It felt like you were in second year at school and you'd been brought into the sixth form common room, and the head boy was having a laugh at your expense," he said.
"It's entertaining, I didn't dislike him. He just doesn't listen and comes in with an idea or makes something shocking up to stir the pot."
Boris' bridges are back in the spotlight
Johnson's bid for prime minster has brought renewed interest in controversial proposed architecture projects from his time as mayor, including the Garden Bridge and so-called Boris Island airport in the Thames Estuary.
Designed by Thomas Heatherwick, the green bridge covered with shops and attractions planned for central London cost £53 million before it was scrapped, £43 million of which was public money.
Johnson fuelled renewed speculation on the Irish Sea bridge idea this week, saying he was an "enthusiast" for the idea at a Conservative Party hustings.
Grimshaw is a London-based architecture studio founded by high-tech architect Nicholas Grimshaw, who won the RIBA Gold Medal in 2019. Earlier this year Nicholas Grimshaw announced that he was stepping down as chairman of the practice after almost 40 years. To mark the occasion we looked back at 10 of his key projects.
Photography is by Paul Raftery unless otherwise stated.