The joint venture will purchase projects from Mainstream's Chilean pipeline, which the developer will continue to manage. The venture will be split 60-40, with Mainstream taking the bigger share.
This is the second collaboration between Actis and Mainstream. Last year, the companies joined forces to win three South African tenders worth 238MW.
Speaking about the deal, Mainstream chief executive Eddie O'Connor said: "This is the ideal platform for Mainstream to accelerate the delivery of our 3,500 megawatts of projects in Chile at a significant scale."
O'Connor said Mainstream could look to replicate the Actis deal in its other markets.
"This platform is about meeting the needs of offtakers, particularly large-scale industrial consumers in Chile who need top-quality projects and competitive electricity prices," he added.
"The market is looking for independent power producers with strong financial backing, expert local knowledge and experience in delivering operational assets. This joint venture very much ticks all of these boxes."
Chile has a large pipeline under consideration, but the renewables sector is facing a watering down of support from the centre-right government of Sebastián Piñera, which wants to reduce its renewables target and remove financial support.
Although Chile added only 17MW to its installed capacity in 2012, taking it to 205MW, it held two tenders to lease public land for renewables. It is taking advantage of the mining sector's need to reduce emissions and the number of projects under construction has soared.