In 2014, Altman was named president of Y Combinator, regarded as the best seed-stage accelerator in the world by its founders as well as the most commercially successful. Altman announced in a 2014 blog post that the total valuation of all Y Combinator companies had surpassed $30 billion, including well-known companies like Airbnb, Dropbox, Zenefits and Stripe.
Altman has stated that he hopes to grow Y Combinator so that it eventually funds 1,000 new companies per year. He has also increased the types of companies funded by YC, especially on so-called "hard technology" companies. 
In July of 2015, Altman announced the YC Fellowship, a new program to give grants of $12,000 and advice to very early-stage startups. 
Altman was named the top investor under 30 by Forbes in 2015, one of the "Best Young Entrepreneurs in Technology" by BusinessWeek in 2008 and listed as one of the five most interesting startup founders between 1979 and 2009 by his colleague Paul Graham.
Pre-YC angel investing
He is an investor in many companies, including Stripe, Shoptiques, Teespring, Zenefits, reddit, FarmLogs, Instacart, Pinterest, Optimizely, Airbnb, Verbling, Soylent, Vicarious, Clever, Notable PDF and Change.org.
He was the CEO of reddit for 8 days in 2014 after the then-CEO Yishan Wong resigned. As part of his investment, he developed a new way for the community to own part of the company. He announced the return of Steve Huffman as CEO on July 10, 2015.
He is Chairman of the Board for Helion and uPower, two nuclear energy companies. He has stated a belief that nuclear energy is one of the most important technological developments for the future.
Guest lectures included Paul Graham, Peter Thiel, Adora Cheung, Alex Schultz, Marc Andreessen, Ron Conway, Carolynn Levy, Kirsty Nathoo, Ben Horowitz, Brian Chesky, Alfred Lin, Aaron Levie, Reid Hoffman, Kevin Hale, Ben Silbermann, and Emmett Shear.
- "Sam Altman".
- "Sam Altman for President".
- "Top Startup Incubators And Accelerators: Y Combinator Tops With $7.8 Billion In Value". Forbes. 2012-04-30. Retrieved 2013-08-05.
- "YC Portfolio Stats". Y Combinator. 2014-07-16. Retrieved 2015-02-13.
- "Y Combinator President Sam Altman is Dreaming Big". Fast Company. 2015-04-16. Retrieved 2015-07-22.
- "YC Fellowship". Y Combinator. 2015-07-21. Retrieved 2015-07-22.
- "Forbes' 30 Under 30 2015: Venture Capital". Forbes. January 5, 2015. Retrieved February 28, 2015.
- "Tech’s Best Young Entrepreneurs". BusinessWeek. 2008-04-18. Retrieved 2009-04-19.
- Graham, Paul (April 2009). "Five Founders". Retrieved 2009-04-19.
- Altman, Sam. "Angel List". Angel List. Angel List. Retrieved 20 April 2015.
- Forrest, Conner. "How Notable PDF built an annotation tool for PDFs on the web". Yahoo! Tech. Yahoo!. Retrieved 20 April 2015.
- "A New Team At Reddit". Sam Altman. 2014-11-13. Retrieved 2015-02-13.
- "An Old Team At Reddit". reddit. Retrieved 2015-07-13.
- "Energy". Sam Altman. Retrieved 2015-07-13.
- "Executives / Loopt". Loopt. Retrieved 2009-04-19.[dead link]
- "Startup Loopt Lands with Green Dot". Wall Street Journal. 2012-03-09. Retrieved 2012-03-13.