Ralston has been at Y Combinator since 2011 and has known cofounder Paul Graham for 20 years.
Ralston says that part of the draw to early-stage investing is " seeing Uber before it's Uber," and watching founder teams build an idea into a mutli-billion-dollar business.
Y Combinator has helped mentor companies like Airbnb, Dropbox, and Stripe.
Y Combinator's new president isn't trying to shake things up, but he's definitely planning on getting a least a little hands-on.
Ralston will replace Sam Altman, who announced his departure as president in March to dedicate his time to OpenAI, the research organization he co-founded with Elon Musk.
With Ralston now at the helm, Altman is also stepping down as chairman, but will remain in an advisory role.