Hickman and Rose solicitors said the guilty verdict was a "watershed" and must mark a turning point for the The Independent Office for Police Conduct(IOPC) and the Crown Prosecution Service(CPS) in how errant police officers are dealt with.
Director of deaths in custody charity Inquest, Deborah Coles, said: “The prosecution of police officers is a rarity in the UK. Today’s guilty verdict reflects the evidence of excessive police violence.
"It is historic and sends a strong message that police are not above the law. However, the prosecution of a few police officers does not address the racism and discrimination embedded in policing.
"Since Dalian’s death the rollout and use of Taser by police has risen significantly, despite the well-known risks these weapons pose to people with mental or physical ill health.
"Dalian’s death is not an isolated case but part of a systemic problem. For decades black men, particularly those in mental health crisis, have disproportionately died following use of force by police.
"True justice requires structural change across our society and its institutions to address racism and respond better to mental ill health and state violence.”
Kate Maynard, of Hickman and Rose, said: “This is the first manslaughter conviction in the modern era of a police officer using excessive force in the course of duty.This is a landmark conviction. I hope it is a watershed moment for accountability of police officers in this country.
"Police officers involved in fatalities have all too rarely faced criminal proceedings; even internal police disciplinary proceedings remain unusual, despite over 25 years of independent investigations by the IOPC and its predecessor.
"It is striking that even before the first anniversary of George Floyd’s death in Minneapolis, the police officer who murdered him had already been convicted.
"By contrast, Pc Monk was able to blame the five-year delay on his vague and variable evidence. And that delay caused Dalian’s family significant anguish and uncertainty. They were forced to wait patiently for almost half a decade to hear the details of what happened on August 15, 2016.
"While the wheels of justice have, in this case, turned far too slowly, today’s guilty verdict must mark a turning point for the IOPC and CPS.
"Until now, they have failed to give the public confidence that police officers who break the law by using excessive force on duty will be held fully accountable.
"That has to change. The CPS need to place more trust in juries and end the past reluctance to prosecute police officers, including after critical findings at public inquiries, inquests or civil proceedings.”
Inquest said that since the former Premier League player's death there had been 103 deaths in or following police custody or contact. This excluded deaths involving police shootings, road traffic incidents or pursuit.
Of these at least 20 were men with black or mixed ethnic backgrounds. In 38 of these cases restraint was used prior to the death. But the charity said the data did not necessarily mean the restraint or use of force caused or directly contributed to the death, only that the person was restrained by police prior to death.