The frontrunners are MPs Dame Tessa Jowell and Sadiq Khan.
Others on the list are MPs David Lammy, Diane Abbott, Gareth Thomas and journalist Christian Wolmar.
The candidates are hoping to take back control of the city-wide mayoralty, which has been held for the past eight years by Conservative Boris Johnson.
The hopefuls secured the minimum required nominations from constituency parties and survived an interview process on 13 June to make it onto the ballot paper.
But the bookies' favourite to succeed Mr Johnson is Conservative MP and environmentalist Zac Goldsmith - who has declared that he will stand so long as he receives the approval of his Richmond Park constituents in a mini-referendum.
Voting, on a one-member, one-vote basis, will be open to Labour members and anyone who pays Â£3 to be a registered supporter or signs up as an affiliate supporter by 12 August.
The victor will be announced alongside the party's new leader and deputy over the weekend of 12/13 September.
Acting Labour leader Harriet Harman said: "The mayor has let down London for too long. We need a Labour mayor in City Hall to stand up for the interests of all Londoners, from Richmond to Redbridge and from Barnet to Bromley."
Ex-Culture Secretary Dame Tessa led the way in nominations from constituency parties - who had two votes each, one of which had to be for a female candidate - with 63.
Mr Khan, who quit shadow justice secretary to concentrate on the mayoral contest, has garnered 42, Mr Lammy 15, Ms Abbott seven, Mr Thomas six and Mr Wolmar five - the minimum number required.
Nominations from trade unions and other affiliated organisations went mostly to Mr Khan with seven while Dame Tessa received two and Mr Thomas one.
Mr Khan said: "I will now spend every day until the polls close convincing Londoners taking part in the selection process that I am worthy of their trust and that I can deliver the victory in 2016 we all want".
The Conservatives have not yet revealed when they plan to run their selection process although three people - deputy mayor Stephen Greenhalgh, London Assembly member Andrew Boff and businessman Ivan Massow - have so far said they are running.
Six candidates will fight it out to be Labour's candidate in next year's London mayoral election, the party has announced.