Altman grew up in St. Louis, Missouri, where he received his first computer at the age of 8. He was raised Jewish and is openly gay. He attended John Burroughs School for high school and studied computer science at Stanford University until dropping out in 2005. He received an honorary degree from the University of Waterloo in 2017.
In 2005, at age 19, Altman co-founded and became CEO of Loopt, a location-based social networking mobile application. Loopt was shut down in 2012 after failing to get traction and was acquired by the Green Dot Corporation.
In 2014, Altman was named president of Y Combinator, whose first batch of investments included Loopt. In a 2014 blog post, Altman said that the total valuation of Y Combinator companies had surpassed $65 billion, including well-known companies like Airbnb, Dropbox, Zenefits and Stripe. In September 2016, Altman announced that he would become president of YC Group, which includes Y Combinator and other units.
Altman has said that he hopes to expand Y Combinator to fund 1,000 new companies per year. He has also tried to expand the types of companies funded by YC, especially 'hard technology' companies.
In October 2015, Altman announced YC Continuity, a $700 million growth-stage equity fund that invests in YC companies. Also in October 2015, Altman announced Y Combinator Research, a non-profit research lab, and donated $10 million to the group. YC Research has thus far announced research on basic income, the future of computing, education, and building new cities.
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.
He is a personal investor in many companies, including Airbnb, Stripe, Reddit, Asana, Pinterest, Teespring, Zenefits, FarmLogs, Shoptiques, Instacart, Optimizely, Verbling, Soylent, Vicarious, Clever, Notable PDF and Change.org.
He was the CEO of Reddit for eight days in 2014 after 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 Oklo, two nuclear energy companies. He has said that nuclear energy is one of the most important areas of technological development.
Altman and Elon Musk are the co-chairmen of OpenAI. OpenAI is a nonprofit whose goal is to advance digital intelligence in a way that is most likely to benefit humanity as a whole, unconstrained by a need to generate financial return.
In fall 2014, he taught a class at Stanford University called "How To Start a Startup". 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. He currently teaches Y Combinator Startup School, a 10-week course featuring guests including Stewart Butterfield, Adam D'Angelo, and Dustin Moskovitz.
Altman occasionally posts to his blog, mostly about startups and technology.
He also wrote Startup Playbook, a distillation of advice that Y Combinator gives to its startups.