Johnson today announced the campaign on the London Eye, on the first day of VisitBritain’s British Tourism Week.
The campaign, "Only in London", is expected to deliver an additional £60 million to the capital’s economy by the end of 2009.
It will launch in the UK at the end of April and be rolled out in Europe and North America over the summer.
The campaign will focus on London’s unique experiences, sights and attractions. Visit London has also launched an "Only in London" list of 100 examples why London is unique.
Harvey McGrath, the chairman of the London Development Agency, said: "The LDA is treating the challenges faced by London's business community in the current economic downturn as its highest priority. The promotion of London is a core part of the LDA's role to promote London's economy.
"We are providing additional funding for Visit London as part of the Mayor's Economic Recovery Action Plan, and attracting additional visitors to London is a highly cost-effective way to provide a vital boost to London’s economy."
Johnson added: "Only in London can you visit the historic Houses of Parliament in the morning, ride the London Eye in the afternoon, and see a wonderful West End show before deciding which of the world’s cuisines to sample in its many restaurants."