YouTube CEO Susan Wojcicki on Thursday said she'll be stepping down as head of the world's biggest online video site. In a post on the YouTube blog, the longtime Google executive said the time is right for her to step aside.

"Today, after nearly 25 years here, I've decided to step back from my role as the head of YouTube and start a new chapter focused on my family, health, and personal projects I'm passionate about," Wojcicki said in the post, which was also shared with YouTube employees. 

Wojcicki will be replaced by Neal Mohan, who previously served as chief product officer at YouTube. Wojcicki said she'll help with the leadership transition and, in the long term, take on an advisory role across Google and parent company Alphabet.

"This will allow me to call on my different experiences over the years to offer counsel and guidance across Google and the portfolio of Alphabet companies," Wojcicki said in the post. "It's an incredibly important time for Google—it reminds me of the early days—incredible product and technology innovation, huge opportunities, and a healthy disregard for the impossible."

Wojcicki was one of Google's earliest employees -- she rented her garage to co-founders Larry Page and Sergey Brin when they were building the search giant in the late 1990s. Before being appointed YouTube CEO in 2014, she was Google's senior vice president of advertising.