There is a disconnect in corporate America. Year after year, companies report that they are highly committed to gender diversity. But the proportion of women in their organizations barely budges. For this to change, companies need to treat gender diversity like the business imperative it is.

This is a key finding of the Women in the Workplace 2018 report, the result of a joint, multiyear survey by LeanIn.Org and McKinsey & Co. that we release today. It’s the largest study of its kind, with four years of data from 462 companies employing nearly 20 million people.

This year’s report shows that women continue to be vastly underrepresented at every level. For women of color, it’s even worse. Only about one in five senior leaders is a woman, and just one in twenty-five is a woman of color. Progress isn’t just slow—it’s stalled.

Women are doing their part. They’ve been earning more bachelor’s degrees than men for over 30 years. They’re asking for promotions and negotiating salaries as often as men. And contrary to conventional wisdom, women are not leaving the workforce at noticeably higher rates to care for children—or for any other reason.

Now companies must do their part, too. That starts with making the business case for diversity, which research shows leads to better performance and more innovation. Then companies need to explain to employees why making a personal commitment to hire, promote, mentor and support women is good not just for business, but for their own careers. Whether CEO or entry-level employee, the person who can work better with half the population will get better results.

Once companies make the case, they need to follow through by reporting on progress and holding managers and leaders accountable for results. Most companies aren’t taking these basic yet critical steps to correct their gender gaps.

Lean In President Rachel Thomas Photo: David Paul Morris/Bloomberg News

To make progress quickly, companies should focus on the two biggest levers: hiring and promotions. As it stands now, women are disadvantaged from the beginning. It’s like they’re running a race and men are given a huge head start. At the entry level, when one might expect an equal number of men and women to be hired, men get 54% of jobs, while women get 46%. At the next step, the gap widens. Women are less likely to be hired and promoted into manager-level jobs; for every 100 men promoted to manager, only 79 women are. As a result, men end up holding 62% of manager positions, while women hold only 38%.

The fact that men are far more likely than women to get that first promotion to manager is a red flag. It’s highly doubtful that there are significant enough differences in the qualifications of entry-level men and women to explain this degree of disparity. More probably, it’s because of performance bias. Research shows that both men and women overestimate men’s performance and underestimate women’s. This may be particularly acute for women at the start of their careers, when their track records are shortest—and for women of color, who are up against both gender and racial bias.

Where Are Women on the Corporate Ladder? A study of 279 companies by LeanIn.Org and McKinsey & Co. tracks women at every level of the corporate chain of command and finds that, despite companies’ efforts to advance women, progress has largely stalled. How much do you know about the state of women at work? Drag the dot to guess the share of women represented in the management pipeline. Source: LeanIn.Org and McKinsey & Co. Women in the Workplace 2018 report

By the manager level, women are too far behind to ever catch up. There are significantly fewer women at that level to promote from within and significantly fewer women with the right experience to hire from the outside. Even if companies want to hire more women into senior leadership—and many do—there are simply far fewer of them with the necessary qualifications. The entire race has become rigged because of those unfair advantages at the start.

Companies need to take bold steps to make the race fair. This begins with establishing clear, consistent criteria for hiring and reviews, because when they are based on subjective impressions or preferences, bias creeps in. Companies should train employees so they understand how unconscious bias can affect who’s hired and promoted—and who’s not. And they should track outcomes to make sure candidates are being treated fairly.

Sheryl Sandberg (above) and Lean In President Rachel Thomas say it’s a red flag—and a sign of performance bias—that men are far more likely than women to get that first promotion to manager. Photo: Lean In

If companies continue to hire and promote women to manager at current rates, the number of women in management will increase by a mere 1 percentage point over the next 10 years. But if companies start hiring and promoting women and men to manager at equal rates, we can nearly close the gender gap in management over the same 10 years. That’s a huge opportunity.

In the past, when companies voiced strong commitment to gender diversity, we celebrated. Now we are impatient. The future of women in the workplace hangs in the balance, and good intentions aren’t good enough anymore. Women are leaning in. Companies need to lean in, too.

Ms. Sandberg is the chief operating officer of Facebook Inc. and the founder of LeanIn.Org. Ms. Thomas is the president of Lean In.