The violence did not arrive without warning. On 11 July, Lei Gai-ji, head of the New Territories section of Beijing's liaison office, had mobilised Yuen Long villages during an inauguration ceremony. Four days later, Legislative Council member Junius Ho issued what many read as an open warning to any protesters planning to enter Yuen Long. On the night before 21 July, BBC and Radio Free Asia reported that Yuen Long residents had been warned by relatives of police officers not to wear black shirts the next day — that some 500 men in white with red bracelets would be gathering to target those associated with the protests. A pro-government Facebook page posted a photo of white-shirted men assembling in the afternoon of the 21st: "the villages are ready," it said. District councillors alerted the police. The police said they had been alerted too, and would deploy enough manpower.

Around 10:30 pm, the men pushed into the station concourse. They attacked commuters indiscriminately — not only those in protest black, but journalists, bystanders, and anyone who happened to be there. A chef who had just finished his shift was beaten despite wearing grey. Legislative Council member Lam Cheuk-ting, who arrived at the station and was also attacked, tried to reach police officers nearby for assistance. Two uniformed officers arrived at 10:52 pm, assessed the scene, and left. Thirty officers arrived at 11:20 pm. By then, the mob had departed. The Yoho Mall adjacent to the station had also tried to call police during the attack and could not get through. At 3:55 am, the men in white began dispersing through and around the police perimeter at Nam Pin Wai Village. None were stopped. Police confiscated dozens of wooden sticks and steel rods from a car park as the mob left.

Police commander Li Hon-man, interviewed at the scene, said on video that he did not know if police were late, and that he had not looked at his watch. The next morning, his contact information and that of 11 other Yuen Long Division officers had been removed from the government directory. Chief Secretary Matthew Cheung apologised five days after the attack, acknowledging that the response had fallen short of public expectations. Police associations promptly condemned the apology, with the Police Inspectors' Association issuing "the most serious condemnation" of Cheung's statement. The pro-Beijing Independent Police Complaints Council later characterised the night's events as a "gang fight"; the district's newly elected democratic councillors called that account a rewriting of history. Journalist Gwyneth Ho, who had been attacked while livestreaming from inside the station, noted that the 21 July attack had been among the most-watched live events of the year. Whatever account anyone chose to tell, the footage existed.

At least 45 people were injured in the attack. In the months that followed, journalist Choy Yuk-ling — known as Bao Choy — produced a documentary for public broadcaster RTHK tracing the vehicles that had supplied the attackers with weapons. To identify the car owners, she accessed a government vehicle licensing database, checking a box for "other traffic and transport related matters." She was arrested in November 2020, found guilty in April 2021, and fined HK$6,000. The Foreign Correspondents' Club called the verdict a dangerous precedent. The Court of Final Appeal exonerated her in June 2023. By July 2021, seven of the attackers had been convicted and sentenced to terms of three and a half to seven years. An eighth was convicted in September 2022. Of 58 people arrested in connection with the attack, 21 were eventually convicted of rioting.

The attack was widely regarded as a turning point. For many people in Hong Kong who had been politically neutral or unengaged, the images from Yuen Long station — and the questions about why police had not been there — eroded what remained of trust in the institutions of the city. Hundreds of thousands came out for a Reclaim Yuen Long march six days later, on 27 July, after police initially objected to the route. On the 21st of every subsequent month, citizens staged sit-ins at Yuen Long station, quietly marking the date. The 2019 district council elections, held that November, saw pro-democratic candidates win 33 of the 39 elected seats on the Yuen Long District Council, flipping a body that had long been dominated by pro-government councillors. Tommy Cheung, a former student leader who was elected to the council, became chairman of the task force established to investigate the mob attack. The Hong Kong Police Force declined its formal invitation to attend the council meeting.