Since it started 11 years ago, The Huffington Post has been synonymous with the personality and the interests of its co-founder, Arianna Huffington.

The pioneering web publication, known for its aggressive use of aggregation and an unapologetically liberal worldview, would go on to win a Pulitzer Prize and expand globally during a challenging time for all media organizations.

But over the last year, The Huffington Post has found itself an increasingly small part of an increasingly large media and tech conglomerate. When Verizon, which announced it had purchased The Huffington Post’s parent company, AOL, in May 2015, bought Yahoo’s internet business last month, Ms. Huffington’s perch at the company seemed increasingly precarious.

With her abrupt announcement on Thursday that she was stepping down as editor in chief of The Huffington Post and leaving the company to focus on her new venture, a health and wellness start-up called Thrive Global, she leaves the publication in an unfamiliar position. For the first time since its founding in 2005, The Huffington Post will be without Ms. Huffington. No successor was named.