Susan Wojcicki, YouTube’s chief executive and one of the most prominent women in Silicon Valley, said on Thursday that she was stepping down from her longtime role leading the internet’s most popular video service.

Ms. Wojcicki’s departure signifies an end to the early, pioneering days of YouTube and its parent company, Google, and adds to a lengthening list of women who have left senior roles at big Silicon Valley companies in recent years.

Google spent some of its early days headquartered in Ms. Wojcicki’s garage. A personal friend of Google’s founders, Larry Page and Sergey Brin, she became one of the company’s earliest employees and climbed its executive ranks over the past two decades. After Google acquired YouTube, Ms. Wojcicki, 54, helped turn the video platform into one of the world’s largest social media companies.

“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,” Ms. Wojcicki wrote in a letter to employees, which was published on YouTube’s official blog.

Neal Mohan, the company’s chief product officer, will take over as chief executive.

Ms. Wojcicki’s decision to step down follows the departures of other prominent women who helped to lead major technology companies. Sheryl Sandberg, who was chief operating officer at Meta, left her role last year. Meg Whitman, who led Hewlett-Packard and one of its successor companies; Ginni Rometty of IBM; and Marissa Mayer of Yahoo have all also left senior roles in recent years.

“Susan has a unique place in Google history and has made the most incredible contribution to products used by people everywhere,” Mr. Page and Mr. Brin said in a joint statement. “We’re so grateful for all she’s done over the last 25 years.”

Andrea Faville, a YouTube spokeswoman, declined to comment on Ms. Wojcicki’s health. But in recent months, Ms. Wojcicki appeared to cut down on public appearances, such as missing a “Made on YouTube” event in September.

Ms. Wojcicki will remain as an adviser at Alphabet, Google’s parent company. Sundar Pichai, Alphabet’s chief executive, asked that she counseled businesses across the company’s portfolio at a time when artificial intelligence was poised to transform many of its products.

“Susan played a key role in Google’s origin story, and over the years her leadership opened up entirely new chapters for the company,” Mr. Pichai said in a statement. He added that there were “exciting opportunities” ahead for the platform.

Mr. Mohan inherits YouTube during a period of challenges. The company is contending with a slowdown in digital advertising that has halted its growth, and it faces increasing competition from TikTok, a Chinese-owned short video service.

YouTube added its own short-video feature, Shorts, to compete with TikTok, and it has tried to become a destination for more premium content by offering cable television programs and the N.F.L. Sunday Ticket, a package of weekly football games.

This is a developing story. Check back for updates.