Susan Wojcicki, who helped turn Google from a start-up in her garage into an internet juggernaut and became one of Silicon Valley’s most prominent female executives with her leadership of YouTube, died on Friday. She was 56.

Her death was confirmed by her husband, Dennis Troper, who wrote on Facebook on Friday that she had been living with lung cancer for two years.

A YouTube spokesman confirmed the date of her death.

Sundar Pichai, Google’s chief executive, announced her death in a statement on Friday. “She is as core to the history of Google as anyone, and it’s hard to imagine the world without her,” he said.

Her more than two decades working with Google began in 1998 in her house in Menlo Park, Calif., the garage of which she rented to her friends Larry Page and Sergey Brin, the company’s founders. For $1,700 a month, the two used the space as their office to build the search engine.

Ms. Wojcicki, who was working at Intel, soon joined Google as one of its earliest employees, as its first marketing manager. Over the years, she reached its executive ranks, becoming Google’s most senior woman employee. She eventually led YouTube, which Google acquired in 2006, to become one of the world’s largest social media companies.

When she became YouTube’s chief executive in 2014, Ms. Wojcicki was hailed as the most powerful woman in advertising. She had made Google enormously profitable and would presumably repeat the trick at YouTube. She led Google’s ad business and played a key role in its acquisition of DoubleClick, an advertising technology company, in 2007.

At YouTube, she introduced new forms of ads and subscription offerings for music, original content and YouTube TV. Her role then shifted to control of hate speech, inappropriate content, extremism and misinformation on the platform as it became the internet’s most popular video service during her tenure.

She stepped down from her role last year but remained an adviser of Alphabet, Google’s parent company. She worked as a philanthropist, Mr. Pichai wrote in a letter to his staff, including supporting research for the disease that took her life. She had built a personal fortune estimated in the hundreds of millions throughout her career.

A full obituary will follow.