As a philanthropist, nonprofit board member and social entrepreneur, I often hear how important high-net-worth donors are. Yes, they are important, as any donor is important. But the real future of the giving space rests with millennials.

Millennials will be the largest demographic in the American workforce by 2020.

There has been plenty written about the influence millennials are having on the workplace. Overall, they desire work-life balance and are increasingly unsettled about the future. In the same way that they are reshaping the workplace, they are reinventing philanthropy.

Today, millennials make up about one-third of the workforce. Gen X is another third and baby boomers are a shrinking 25%. By 2020, millennials will represent more than half of all workers. That’s a monumental and rapid shift — and it’s all taking place right now.

As millennials begin to increase disposable income, pay student loans and acquire patterns and behaviors that are more likely to last into their forties and fifties, what was once a finicky, trend-receptive audience is becoming more grounded, regular and even predictable...