Kan was a member of the first batch of YC-funded startups in 2005 for Kiko Calendar, and was funded by YC again for Justin.tv and Exec. Kan became a partner at Y Combinator in March 2014, where he offered advice to the new startups in each batch. In March 2017, Kan left Y Combinator to start his own incubator, Zero-F.

In 2007, Justin Kan and partners Emmett Shear, Michael Seibel and Kyle Vogt started Justin.tv, a 24–7 live video feed of Kan’s life, broadcast via a webcam attached to his head.

Afterward, the company transitioned to providing a live video platform so anyone could publish a live video stream. Justin.tv, the platform, launched in 2007 and was one of the largest live video platforms in the world with more than 30 million unique users every month.

After Justin.tv launched in 2007, the site quickly began building subject-specific content categories like Social, Tech, Sports, Entertainment, News & Events, Gaming and others. Gaming, in particular, grew very fast and became the most popular content on the site.

The company then decided to spin off the gaming content under a separate brand at a separate site. They named it TwitchTV, inspired by the term twitch gameplay. It launched officially in public beta on June 6, 2011.

Justin Kan launched Exec on February 29, 2012, a new service to allow anyone to outsource anything you want for $25/hour. Exec was co-founded with his brother Daniel Kan, former head of UserVoice business development, and Stanford graduate Amir Ghazvinian.

Justin.tv was closed on August 5, 2014, in an effort to focus further on Justin.tv’s parent company, Twitch.

Twitch was acquired by Amazon.com in August 2014 for $970 million.

Exec was purchased by Handybook, a company founded by Oisin Hanrahan, Umang Dua, Ignacio Leonhardt, and Weina Scott, in an all-stock transaction in January 2014.

Inspired by his frustration at being a power user of legal services, Kan publicly launched Atrium in 2017 with the vision of building a technology-first law firm focused on offering legal services and fundraising advice for startups powered by machine learning. Kan raised $10.5 million in an initial “party” round of investment led by General Catalyst In September 2018, Kan raised a $65 million funding round led by Andreessen Horowitz. At that time, Andrew Chen, Marc Andreessen and Michael Seibel joined the Atrium board of directors. Atrium has closed operations from March 2020.