The deal, which is subject to shareholder approval, marks the single largest equity investment in Mainstream's five-and-a-half-year history. It gives Marubeni, whose global renewables interests extend to the UK's electricity supply business, the right to be represented on the Mainstream board.
Barclays, which invested in Mainstream in 2008, also has board representation.
Mainstream and Marubeni have a long-term strategic alliance to accelerate the former's key projects around the world, which include a proposed 450 megawatt (MW) offshore wind farm 15km off the coast of Fife.
Mainstream recently secured onshore planning permission for the Neart na Gaoithe project in the Forth Estuary, which will generate enough renewable power to supply 335,000 homes.
Chief executive Eddie O'Connor said: "This investment is a game-changer for Mainstream, allowing us to focus on accelerating our project portfolios across a range of markets as well as entering into new jurisdictions which present strong value opportunities for our business.
"Having grown our company in the midst of the global financial crises, this type of long-term strategic investment is true testament to the strength and experience of our team as well as the quality and spread of our global project portfolio."
Since 2008 Mainstream has developed a global pipeline of more than 19,000MW of wind and solar projects in four continents, with plant in construction and in operation in Ireland, South Africa, Chile and Canada. It signed a $1.4 billion deal with Actis to develop a wind and solar platform in Chile, received a grid connection from National Grid earlier this year for its Energy Bridge that will allow it to export 5000MW of wind energy from Ireland to the UK, starting in 2018.