She said she decided to leave the online news organisation, which now operates in 10 languages and has a user base of over 200 million, because she "couldn't do justice to both companies."
"I'm filled with excitement at the prospect of devoting the rest of my life to accelerating the culture shift away from merely surviving and succeeding to thriving," she said.
Thrive, which will launch after the US election in November, has been funded by Lerer Hippeau Ventures and other groups and individuals including NBA star Andre Iguodala and entrepreneur and philanthropist Sean Parker.
The Huffington Post was among the first major news organisations to benefit from consumer interest in online information.
Today, in addition to the US edition, the HuffPost has English language websites for British, Canadian and Australian readers, a number of French-language editions and versions in German, Japanese, Greek, Portuguese, Arabic and Korean. It has said it plans to launch in China as well.
- Additional reporting: AFP