Labour announced on Twitter at lunchtime that Khan had won the six-way contest ahead of nearest rival Tessa Jowell. Khan's Conservative opponent in the May 2016 contest will be announced later this month - with Zac Goldsmith seen as the favourite.
Andy Sawford, CEO of Connect Communications and a Labour MP between 2012 and 2015, said he knows Khan well and called him a "great and energetic campaigner" with a "lot of substance" to his policies.
"What Sadiq has shown is he can reach across different parts of London; he has done that in the selection process," he said.
Jack Taylor, political consultant at The Whitehouse Consultancy and a former Labour researcher, said: "As the son of a migrant bus driver, raised on a south London council estate, his story encapsulated the capital’s dynamism and opportunity. However, it also reaffirms the Labour’s ongoing reorientation and a line under Blairism as an electoral force."
Khevyn Limbajee, a senior consultant at Cavendish Communications and Labour supporter, said of former human rights lawyer Khan: "I think he has got a great backstory, I think that is a powerful symbol of the British dream. His support looks broad across the party."
Luke Pollard, director of Corporate Reputation Consulting and an unsuccessful Labour candidate at the last general election, said: "What Khan has been able to do is successfully position himself in a crowded field to the left of his major challenger. His big challenge now is to make a distinctive offer to what Zac Goldsmith is expected to make at the end of the month."
All four also referenced tomorrow's big announcement of the party's new leader. The expected victor is left-wing candidate Jeremy Corbyn, with Sawford saying today's result makes this seem ever more likely.
While there have been suggestions that success for Corbyn would create division within the party, Pollard said he expected a "united Labour party in London", commenting that "they will have to be more united due to electoral realities". Limbajee said he expected a Goldsmith-Khan race to be "a tough fight and an interesting contest".
Taylor said: "Labour’s left should not, however, get too comfortable – should Khan fail in 2016 the blame is likely to fall at new expected leader Jeremy Corbyn’s door and could even prompt manoeuvres against him."
PR professionals - from both sides of the political spectrum - voiced their opinion on Twitter.
Labour Party slowly killing itself. Not fit to govern, not fit to oppose. It just doesn't represent the feelings of modern Britain.— David Roach (@davidroachcouk) September 11, 2015
I have no words. All I have is utter despair. https://t.co/8DPMWWdRsg— Jo-ann Robertson (@RedRobertino) September 11, 2015
As had been the case following May's general election, the role of opinion polls has also come under question.
Interesting that polls had Jowell ahead in the #LabourMayor contest, but Khan won. Have the polls misled on the leadership contest too?— Westminster Advisers (@WA_Comms) September 11, 2015
Yesterday, Khan thanked his campaign team in a Tweet using the Obamaesque hashtag #TogetherWeKhan, which he has used throughout the campaign.