Apple CEO Tim Cook publicly came out as gay in an editorial published by Bloomberg Businessweek on Thursday. Cook said he hopes to help move civil rights forward. (Reuters)

When Apple chief executive Tim Cook said Thursday that he is “proud to be gay,” he did not only become the first openly gay leader of a major U.S. company. He also swept his obsessively private company into the forefront of one of America’s most public movements, cementing its place in the debate over equality in the workplace and beyond.

Apple, the world’s most valuable company, is famously and fanatically tight-lipped, having long neglected to tell its shareholders about former chief executive Steve Jobs’ cancer diagnosis. But under Cook, the tech giant is now taking on bigger, bolder advocacy roles in social policy and politics, eschewing the corporate America convention of avoiding touchy subjects and winning acclaim from its customers in turn.

A company once rarely seen in Washington, Apple has in the past year spoken out in support of a workplace-equality bill before Congress, advocated for same-sex marriage in California and opposed a bill that allows discrimination against gays and lesbians in Arizona. This week, Cook urged Alabama, his home state, to more actively protect gay rights.

“We’ll continue to fight for our values, and I believe that any CEO of this incredible company, regardless of race, gender, or sexual orientation, would do the same,” Cook wrote in an essay Thursday for Bloomberg Businessweek. “And I will personally continue to advocate for equality for all people until my toes point up.”

Cook’s sexual orientation was widely known in Silicon Valley, but he did not make his essay announcement on his own. He first sought support from Apple’s board, which Chairman Arthur Levinson said in a statement gave its “wholehearted support and admiration.”

Cook’s coming out in the corporate suite was unprecedented. Before his announcement, of the chief executives in the country’s 500 largest public companies, only 5 percent were women, 1 percent were black — and zero were publicly gay or lesbian, said Kenji Yoshino, a professor of constitutional law at New York University who last year co-authored “Uncovering Talent: A New Model of Inclusion.”

“It’s enormously significant,” Yoshino said, “precisely because there’s been a big gap between what middle management in the Fortune 500 has looked like, and its leadership, and what it looks like.”

The lack of openly gay leaders has long been a puzzle for the gay and lesbian community. One hypothesis, Yoshino said: Workers who keep their sexual orientation private when joining a company might find it hard to come out as they rise up the ranks — a tension that Cook’s candor may help ease.

“By this action, he is not going to just increase the visibility of inclusiveness in the workplace,” said Sylvia Ann Hewlett, an economist and founder of the Center for Talent Innovation, a diversity think tank. “He will give a lot of companies the courage to join in.”

Same-sex marriage, which is now legal in 32 states and the District of Columbia, has never been more accepted nationwide. About 52 percent of Americans approve of gay marriage, compared with 40 percent who oppose, Pew Research shows. Young Americans of the iPhone generation are even more accepting: About 67 percent of millennials, or those born after 1981, are in support.

Yet there are still plenty of risks to being gay in the American workplace. A Human Rights Campaign study this year found that 53 percent of gay, lesbian, bisexual and transgender workers hide their sexual orientation, largely because they fear how their co-workers will react. And workers can be fired for doing exactly what Cook did in the 29 states that lack employment non-discrimination laws covering sexual orientation, American Civil Liberties Union data show.

“There’s no question that even in 2014 it is hard for people to come out — it’s hard to be open about who you are,” said Charles Joughin, spokesman for the Human Rights Campaign. “For one of the, if not the, most prominent CEOs to say today, ‘I’m gay, and I’m proud to be gay,’ is a monumental step.”

Cook’s announcement is remarkably different from that of John Browne, the first top executive of a Fortune 500 company to publicly come out as gay. In January 2007, Browne, who had served as chief executive of BP for 12 years and was a closeted gay man, was wrenched into the spotlight after a former boyfriend told a British newspaper about their relationship. Browne tried to block the story’s publication, telling the court he’d met the man while jogging rather than through an escort agency. He resigned in May 2007 in the wake of the scandal.

Browne, who this year wrote “The Glass Closet: Why Coming Out is Good Business,” said Cook’s coming out was “an important step in the journey towards full and uncompromising inclusion of LGBT people.” In a June essay, Browne said that after he resigned he received thousands of supportive letters from across the world, adding, “Had I known then what I know now, I would have come out sooner.”

Cook’s announcement marks only the latest degree of separation from Jobs, his cagey predecessor who acquired a cult following even while shirking the public limelight. Cook has written high-profile editorials and spoken openly in media interviews, including last month during a two-part series with talk-show host Charlie Rose, but “the only time Steve Jobs did press stuff was for a product announcement,” said Ken Auletta, a New Yorker writer who has reported extensively on Jobs. “Otherwise, he was very reclusive.”

Apple has long been associated with protections of sexual orientation and gender identity in the workplace, and it is one of the few companies to score the top rating on the Human Rights Campaign’s Corporate Equality Index every year since 2002.

But Apple’s calls for openness outside its headquarters have never been so visible. In February, Apple joined other companies in urging Arizona Gov. Jan Brewer (R) to veto a bill allowing businesses with strong religious ties to deny service to gays and lesbians. The call came only a few months after the tech giant had announced it would build a sapphire glass plant in Mesa, Ariz., creating 2,000 jobs. The bill was quickly vetoed.

In his essay, Cook cited civil rights leaders Martin Luther King Jr. and Robert F. Kennedy and said he hoped his coming out would in some way contribute to a greater good. “We pave the sunlit path toward justice together, brick by brick,” he wrote. “This is my brick.”

Andrea Peterson and Cecilia Kang contributed to this report.