Time’s Up, the advocacy organization founded by powerful women in Hollywood as the #MeToo movement roiled the nation, announced on Monday that it had appointed Tina Tchen, a lawyer and former chief of staff to Michelle Obama, as its new chief executive.

Ms. Tchen was a founder of the organization’s signature initiative: a legal-defense fund for women in all industries who experienced sexual harassment at work. The fund has so far raised more than $24 million.

The previous chief executive, Lisa Borders, resigned abruptly in February, after less than four months on the job, after her adult son was accused of sexual misconduct. Rebecca Goldman, who had served as interim chief executive, will continue as chief operating officer.

Ms. Tchen, 63, called it “the role of a lifetime” after more than 30 years of working on diversity and inclusion. She said it was a pivotal moment in which those issues had come to the forefront around the world.

“We are having a global conversation around workplaces, around the role of women and gender equity, in a way that in those three decades, I’ve never seen before,” she said on Friday.

The organization is also starting a new project called the Time’s Up Impact Lab, focused on research and policies about sexual harassment and other forms of workplace discrimination. The lab received a significant contribution from Pivotal Ventures, an incubator founded by Melinda Gates and led by Jennifer Klein, a strategist for Time’s Up.

The organization declined to say exactly how much Pivotal Ventures had donated. In an article in Time, Ms. Gates pledged to donate $1 billion over the next 10 years to expand women’s power and influence in the United States. She wrote that the group would focus on dismantling barriers to women’s professional advancement; fast-tracking women in sectors such as technology, media and politics; and pressuring companies and organizations to change policies.

Time’s Up was founded two years ago amid the fallout from revelations about Harvey Weinstein and other powerful Hollywood figures. Actresses, producers, agents and other women began to meet and strategize about a campaign for gender parity and workplace safety. They were prompted to think about women in other industries by an open letter from female farmworkers with the group Alianza Nacional de Campesinas that expressed both sympathy and solidarity.

Time’s Up was a formidable presence at the Golden Globes in January 2018. Members invited activists as their guests, and most celebrities attending the awards show wore black as a sign of solidarity. Many added pins with the Time’s Up logo.

The legal defense fund was started the same month, and since then, it has connected more than 3,600 workers in dozens of industries to legal or public relations support for sexual harassment cases. They have included farmworkers, wait staff, F.B.I. agents and opera singers. Most had low incomes, the organization said.

The fund is administered by the National Women’s Law Center. Major initial donors included Shonda Rhimes, Reese Witherspoon and Meryl Streep. Nearly 800 lawyers have volunteered free or low-cost legal services.

The other co-founders of the fund are the lawyer Robbie Kaplan; Fatima Goss Graves, the president and chief executive of the National Women’s Law Center; and the communications strategist Hilary Rosen.

Ms. Kaplan, who is best known for representing Edith Windsor in the Supreme Court case that forced the government to recognize same-sex marriage, said in a statement that Ms. Tchen “is like my long-lost sister — someone I have looked up to since we first worked on a case together years ago.”

While pursuing individual cases, the organization also tried to work with lawmakers and others around the country to amend or introduce legislation related to sexual assault and workplace discrimination. Perhaps its most notable victory so far was a bill signed by Gov. Andrew M. Cuomo of New York last month to extend the statute of limitations in certain rape cases.

Mr. Cuomo was joined at the signing by Ms. Kaplan and actresses affiliated with Time’s Up, including Julianne Moore, Mira Sorvino and Michelle Hurd.

The organization has also pushed studios to hire more women directors, producers and executives, and has created projects focused on other industries, like health care and advertising.

Ms. Tchen currently leads Buckley LLP’s workplace culture practice, and will leave the firm to become president and chief executive of Time’s Up on Nov. 1. She will continue as a chairwoman of the group The United State of Women, along with Valerie Jarrett, her former colleague in the Obama White House.

At the White House, Ms. Tchen headed the Office of Public Engagement and the White House Council on Women and Girls, and served as an assistant to the president and chief of staff to the first lady. Before that, she was a longtime lawyer and partner at the corporate firm Skadden, Arps, Slate, Meagher & Flom. In an interview, she made a point of mentioning that she had also been a single mother throughout her career.

Ms. Tchen also serves on several advisory boards, and was asked to help two high-profile organizations after gender-related controversies in the last two years. The first was the Recording Academy, which presents the Grammys. Its previous president, Neil Portnow, came under fire for saying that women in music should “step up” to advance their careers, though the vast majority of nominees are male. Ms. Tchen served as chairwoman of a task force on inclusion and diversity for the organization.

In March, she was asked to be an adviser to the Southern Poverty Law Center after accusations of sexual harassment and racism surfaced at the civil rights nonprofit. Ms. Tchen said both projects were wrapping up.

She will continue to be based in Chicago, where she said Time’s Up would open an office.

In a statement, Ms. Rhimes and the actress Eva Longoria, both principal founders of Time’s Up, praised Ms. Tchen for her dedication and vision.

“She has been with us since the beginning, honoring our commitment to transform work for women everywhere,” Ms. Longoria said.