Johnson and Lebedev, joined by Evening Standard editor Sarah Sands, spent a night on Gresham Street not far from St. Paul's Cathedral.
"People who have served our country in the most difficult and unimaginable situations deserve all the help we can give them," Johnson was quoted as saying by The Independent.
"We've seen compassionate London at its best tonight: brave care workers who care about the people no one else does, and show genuine courage on a daily basis," Lebedev was quoted by The Independent as saying.
The Independent and the London Evening Standard's joint Homeless Veterans campaign is aimed at raising money for former military personnel who are in need of housing not only in London but all over the UK.
According to the Homeless Veterans website, there are about 4.5 million veterans in the UK, with 20,000 individuals leaving the country's Armed Forces annually. In 2013-214 the campaign organizers received 3,400 calls for help.