Following his departure from Google, Buchheit joined Y Combinator (YC), a prestigious startup accelerator, as a partner. He mentored numerous startups, sharing his experience and knowledge with aspiring entrepreneurs.

While at Y Combinator, Buchheit also actively participated in angel investing, providing funding and guidance to promising early-stage companies. He identified and supported numerous successful ventures.

Buchheit co-founded FriendFeed, a social aggregator that allowed users to consolidate updates from various social media platforms into a single feed. This was an early precursor to features seen in modern social media platforms. Bret Taylor (later CTO of Facebook) was another co-founder.

Facebook acquired FriendFeed, demonstrating its value in pioneering social aggregation. The acquisition also brought key talent, including Bret Taylor, to Facebook.

After the acquisition of FriendFeed, Buchheit continued to be actively involved in the startup community. He participated in various investment rounds, mentored founders, and shared his insights on building successful technology companies. He has been involved in a number of undisclosed startups.