The pair will be tasked with leading initiatives that combine entrepreneurship with solving social issues.
Milne, who is finishing up as a management consultant at financial services firm EY, said she was attracted to the idea of bringing "profit drivers and social drivers together".
"I kind of saw that as a huge opportunity or potential for New Zealand and something I thought I really want to be a part of."
Milne said she was interested in helping improve youth voter engagement and also focused on "understanding the limitations for children in New Zealand".
Winstone, a qualified lawyer who has worked in investment banking, said he was drawn by the opportunity to create "some longer-term value".
"This 'people, planet and profit' sort of mantra is not new but I think it's one whose time has come," he said.