He said allowing long-term illegal immigrants to earn the right to stay in Britain would see "hugely increased" tax revenues.
Mr Johnson added: "What I want to do is to commission a study by my own economics team here at the Greater London Authority into the possibility.
"We want to look in detail at what the economic impact of such an earned amnesty system would be."
Mr Johnson’s comments risk opening a rift between himself and Tory leader David Cameron, with whom he openly clashed on the issue of an amnesty earlier this year.
Mr Cameron said: "The problem with amnesties is that they just store up another for the future, as people expect another one."
Of the 700,000 thought to be working illegally in the UK, about 400,000 are in London.
Mr Johnson insisted he did not want to incentivise illegal immigration and said only long-term immigrants would be allowed to stay.
He went on: "There’s got to be a very substantial period in which they have been in this country.
"I think that we could have other hoops that they might have to go through in order to be able to qualify for an earned amnesty scheme. For instance, it might be necessary to have a clean criminal record.
"It might be important that they should go through various citizenship tests, the kind we already have. And there might be some sort of financial obligations that they have to meet as well."