During high school, he spent a semester at the Mountain School in Vermont, where his concern for “what was happening on a planetary scale” crystallized; at Yale, he designed his own major in sustainable economic development, and then took a leave that became permanent to build the Energy Action Coalition, which describes itself as “a coalition of 50 youth-led environmental and social justice groups.”

“Young people had been at the forefront of every major social movement in history, and almost nothing was happening with young people and climate change at that point,” he said. By 21, he was managing a $5 million budget and a staff of 80 in the United States and Canada.

That work led to an advocacy effort that Mr. Parish said helped inspire President Obama’s green jobs programs, as well as a book, “Making Good: Finding Meaning, Money, and Community in a Changing World,” which he wrote with Dev Aujla, a founder of the charitable organization DreamNow.

But it was a stint working at the Black Mesa Water Coalition in Flagstaff, Ariz., starting in 2007, that brought together Mr. Parish’s entrepreneurial vision and personal network. It was there that he worked alongside his wife, Wahleah Johns, to shut down coal plants that had fouled the water supply in the Navajo Nation.

He also reconnected with Dan Rosen, now Mosaic’s chief executive, who had met Mr. Parish while trying to get a solar array for his high school in Ridgewood, N.J., and had gone to work with Black Mesa after graduating.

“That was one of the big, kind of ‘Aha!’ moments about what would become Mosaic,” Mr. Rosen said, referring to discussions about developing solar energy as part of the transition after closing the coal plants. Navajo residents, he said, “wanted to own the projects — they wanted to be owners, they wanted to participate, they wanted jobs, they wanted to invest, even.”

Based in Oakland, Calif., Mosaic is trying to capitalize on that desire, finding and vetting as many projects as its staff of 22 can handle while it raises more money so it can expand. Mosaic makes loans only to projects that already have deals to sell the electricity they will produce; it then raises money from investors, who receive a return of roughly 4 to 6 percent as a loan is paid back.