LinkedIn profile photo of Geoff Ralston, Y Combinator partnerThe newest member of Y Combinator’s team is angel investor and successful entrepreneur Geoff Ralston.

Y Combinator founder Paul Graham announced the addition of Ralston in a blog post yesterday, writing that Ralston is “a perfect match for YC because he’s smart and energetic, and yet informal and a super nice guy.”

Graham told VentureBeat via email that Ralston will be a full partner in Y Combinator, but will continue to be a partner in Imagine K12, the educational technology incubator Ralston founded. Like other Y Combinator partners, he won’t take any board seats in the incubator’s portfolio companies.

Y Combinator is a startup incubator and investment fund that has produced many success stories including Dropbox, Reddit, Scribd, AirBnB, and The organization typically invests a small amount of money and puts the companies it selects through a three-month training program. It has been so successful that SV Angels, DST, and Andreessen Horowitz have together pledged to put a combined $150,000 into each company chosen for the program. In addition, Sequoia Capital is a limited partner and lead participant in Y Combinator’s fund. Graham says that Y Combinator has invested in 383 companies since it started in 2005. It just started accepting applications for its summer 2012 funding cycle; applications are due March 28.

Ralston has been an angel investor for the past several years and also invested in Y Combinator’s $8.25 million fund in 2010.

Ralston has a deep history in Silicon Valley, going back to the pre-PC days. He has a degree in computer science from Dartmouth and worked for Hewlett Packard in the 1980s on the HP 2000 and HP 9000 systems. In 1994, he left HP to work on an internet startup that eventually became Yahoo purchased Four11 in 1997, turning its core product, RocketMail, into the foundation for Yahoo Mail. After nine years at Yahoo, Ralston left to do angel investing, then took the chief executive job at, which eventually sold to Apple in 2009. He’s been angel investing as a “hobby” since then, typically investing an average of $50,000 in very early-stage companies.

He wrote in a 2010 interview that angel investing was more of a hobby than a vocation for him:

I’m not really trying to earn my keep doing this. I do it because I love working with entrepreneurs. I love working with people who are passionate and who have an idea and who want to change the world. And I think it’s usually important for Silicon Valley and for the world and for the economy. I just really like it.

Ralston said in 2010 that he got much of his deal flow from Y Combinator events, so the move to partner is probably a natural one, and was in the works for awhile.

Also in that interview, Ralston indicated that Lala was his most lucrative exit. The Lala sale was considered a “fire sale” at the time, but apparently some investors made out okay.