After graduating from Stanford, Altman joined Y Combinator as an entrepreneur-in-residence in 2005. He worked closely with Paul Graham, the founder of Y Combinator, and helped to develop the company's investment strategy. In 2009, Altman became the CEO of Y Combinator, a position he held until 2014. During his tenure, the company invested in over 1,000 startups, including Airbnb, Dropbox, and Reddit. Altman was instrumental in shaping Y Combinator's investment strategy and helped to establish the company as one of the most successful startup accelerators in the world.