While at Yale, he co-founded Catalyst Recruiting, an online forum for college students to network with potential employers. He took a leave of absence to pursue Catalyst Recruiting, but the company soon withdrew. After graduation, Ries moved to Silicon Valley in 2001 as a software engineer with There, Inc. He worked with the company until the 2003 launch of its web-based 3D Virtual World product, There.com. The company failed soon after.
In 2004, Ries left to join one of There.com's founders, Will Harvey, in co-founding IMVU Inc., a social network. IMVU investor Steve Blank insisted that IMVU executives audit Blank's entrepreneurship class at UC Berkeley. There Ries took Blank's rapid customer feedback method, which Blank called "customer development," and applied it at IMVU in combination with lean software development, testing alternative versions of the product and measuring download rates. IMVU rolled out the code for production nearly 50 times a day, an unusually fast development cycle. Ries also edited an earlier version of Blank's book on customer development, "The Four Steps to Epiphany."
IMVU aimed to integrate instant messaging with a high revenue per customer of traditional video games. Ries and Harvey did not seek a large amount of seed funding and launched a minimum viable product in six months. In 2006, the company raised $ 1 million in its first round of Seraph Group venture fundraising, eventually raising an additional 18 million. In 2008, after a new CEO joined IMVU, Ries resigned as CTO, remaining as an observer on the board.
After leaving IMVU, Ries joined venture capital firm Kleiner Perkins as a venture advisor, and six months later began advising startups independently.
In 2015, Ries began organizing The Long-Term Stock Exchange (LTSE), which aims to align the interests of companies and long-term investors and enhance the experience of public companies. Ries had proposed the LTSE idea in his book The Lean Startup. On November 30, 2018, LTSE submitted an application to the Securities and Exchange Commission for registration as a national stock exchange. On May 10, 2019, the Securities and Exchange Commission approved it.