The new MMC London Fund is designed to provide early stage finance for fast-growing companies and will attract of £10m of City Hall-controlled funding.
Equity fund manager MMC Ventures won a competitive tender to run the venture and will have to find an additional £12m for the total funding pot.
Johnson said: “It is a top priority for me that these entrepreneurial enterprises receive practical support to thrive and grow, not least as many have the potential to become major employers in the future.
“This innovative equity fund is using public and private money in order to offer much needed financing when more traditional routes are not available,” he added.
The first businesses to receive financing from the fund include mobile ticketing company Masabi, holiday home exchange Love Home Swap, and freelance consultants network MBA & Company.
Bruce Macfarlane, managing partner at MMC Ventures, said: “We are delighted that the Mayor has backed the MMC London Fund. We’re looking to back entrepreneurs who will be leaders in their industries and London is brimming with talent.”