The so-called London Green Fund will be used to try to attract more money from sources like the EU, the private sector, charities and philanthropy groups, he said.
The cash would be targeted at local authorities and other public groups and aimed to boost the low-carbon economy and create thousands of green-collar jobs.
"We want to provide an easy way to tackle the maze of complex red tape that acts as a deterrent and provide the upfront funds needed to take action," Johnson said.
A document published by the mayor's office last week, called Leading to a Greener London: an Environment Programme for the Capital, featured a series of pledges including one to encourage more green roofs and green walls.
London Development Agency director of project delivery Martin Powell said the fund was due to be launched at the end of the current financial year.