The one constant almost since its doors opened is 56-year-old Sid Khan, Sadiq's eldest brother and head coach at Earlsfield for the past 30 years.
The Khans come from a family of eight siblings -- seven of them brothers -- whose father Amanullah was a toolmaker in the Pakistani military.
He was deployed to Australia before immigrating to London and working as a bus driver on the No. 44 route for a quarter of a century. To this day, the route runs past the Henry Prince housing estate in nearby Wandsworth, where the kids grew up.
Each of the brothers took their licks at Earlsfield, with all apart from Sadiq having boxed competitively on the amateur circuit.
Sid walked in to the club as an 11-year-old and never left.
"We were all naughty kids there, so it was just one of those things that you did," he tells CNN before overseeing a bustling training session.
"If you didn't belong to boxing, you were part of the army cadets. Back then, it was one or the other. It's not like now -- kids have plenty of things to do."
"I'm the oldest, so I made sure they learned the art of boxing," Sid says of his brothers. "Every one of them has come through this gym. So we've all got it in our blood."
Life skills and upsets
That includes Sadiq, who worked as a human rights lawyer before winning elections as a local councilor, a Member of Parliament and, this year, as the first Muslim mayor of a major Western capital city.
Though his talents lie outside the ring, the 45-year-old still visits the boxing club often, where his images are among those adorned on the walls. The mayor still relishes the sport, and is lobbying for London to serve as the global hub of championship bouts
"The skills you learn are life skills: Being magnanimous, what to eat, how to keep fit, how to look out for each other," he told the New Statesman
earlier this year.
"You've got to defend yourself. We all boxed, and that gives you confidence if you get into bother on the street."
But despite his enthusiasm, Sadiq was never going to be a prizefighter, admits brother Sid: "He never competed but he's done all the training. We won't say that he was going to be the next Olympian, because he wasn't."
Though it could be argued, however, that the son of a foreign-born bus driver defeating Zac Goldsmith -- an Eton-educated billionaire's son -- in the mayoral race was as big an upset as Buster Douglas knocking out Mike Tyson in Tokyo.
Colorblindness in the ring
Earlsfield is a microcosm of London, encompassing people from all ethnic backgrounds and social classes -- a fitting starting point for Britain's highest-ranking Muslim politician.
Although Sadiq's London election victory garnered global headlines, at least in part due to his background, boxing operates on an equal plane, says Sid -- which incidentally is short for Zahed.
"Joe Joyce is half African and half English and he's the lad that had the union jack (flag) around him," he points out about the 6-foot 6-inch, 240-pound super heavyweight who fought for Great Britain in Rio. Joyce's parents are from Nigerian and Irish backgrounds.
"Once you're in the ring, it goes out the window then," he shrugs. "You're boxing for your country. It doesn't matter who you are."
Twenty minutes into this interview, as if by design, aspiring boxers aged 14 to 40 and of all shapes, sizes and ethnicities, flow into the gym; they include a few women. Heavy bags swing in all directions while pairs square off in the ring.
Some look like future professionals, others like kids who are destined for office jobs. Not one of them, however, looks anything like Joyce, whose rise to the top is almost as unlikely as Sadiq Khan's.
Strange journey to Rio
Joyce, from nearby Putney, got his first taste of boxing as a 22-year-old at Earlsfield. The club's trainers, led by Khan, molded him into a European and Commonwealth champion.
He joined a lineage of Earlsfield amateurs who went on to pro fame, chief among them former WBC heavyweight champion Bruno, along with former middleweight champion Richard Williams and current welterweight belt-holder Bradley Skeete.
Though he was a natural athlete growing up, Joyce struggled to find his sport. He tried his luck at martial arts, rugby, swimming, athletics and even cheerleading
before giving boxing a go at an age where his peers were already turning pro.
Ask Sid what he saw in Joyce when he first walked into Earlsfield and his answer is instant: "Dedication -- you tell him to do something and he does it."
Joyce was drilled for a full year before being allowed to compete outside the gym, and it soon became evident that he could hold his own.
The 31-year-old couldn't quite pull off the dream finish in Rio, however, losing to France's Tony Yoka after a controversial decision in the gold medal bout.
It was the last of an array of questionable decisions by the Olympic boxing judges, the worst of which saw Ireland's world champion Michael Conlan lose after bludgeoning Russian Vladimir Nikitin.
Five judges were "transitioned" by amateur boxing's governing body AIBA
shortly after that result, though controversy endured.
"As much as you jump up and down, you're not going to change the ref's decision," Sid says, still rattled by it all. "Joe got shafted, but he took it like a man.
"When you get two runners running, you know who the winner is. Well in boxing it's not like that; it's often down to the decision of the judges."
In response, an AIBA spokesman told CNN: "AIBA stands to the values of respect, sportsmanship and excellence, and remains committed to a fair and transparent sport.
"AIBA will continue to defend the referees and judges community, whose integrity is constantly put into question."
Local boy goes global
British Olympic fighters are assigned corner men pulled from the country's boxing authority, leaving Sid and his fellow coaches to watch Joyce's final bout back in Earlsfield with a TV film crew on hand.
Despite everyone's disappointment, Joyce came home to a street party thrown by his mom to celebrate.
Sid saw it as another achievement for both his small club and the community where he has lived his entire life.
Although he is a mechanic by trade, owning an auto repair shop in the area, his passion lies in volunteering at Earlsfield four days a week.
Most of the time another Khan is there with him. David, one of his two sons who became boxers, was instrumental in training Joyce.
Sid's youngest brother Populas still boxes occasionally but is on coaching duty this evening, holding pads while imploring his prodigies to punch away.
The way things are going, one of them may just blossom into Britain's next Olympic champion -- or perhaps even its future prime minister.