By John D. Stoll

Business is showing signs of getting more serious about diversity. Age-old brands are being shelved. Companies including Bank of America, Cisco Systems and Lego have announced pledges totalling more than $1.7 billion to causes that combat racism. Facebook Inc. has expanded the chief diversity officer role. Google promised to increase "underrepresented groups" in leadership.

General Motors Co. has a hard, worthwhile lesson to teach executives who want to make real changes to their culture. It takes stamina.

GM is a top 20 performer in diversity and inclusion among S&P 500 companies, according to a 2019 Wall Street Journal analysis. A third of its U.S. hires in 2018 were minorities; 40% of C-Suite jobs belong to minorities and women, including Chief Executive Mary Barra.

It took nearly a half-century of achingly slow gains and painful failures to get there.

GM's journey started in 1972, when Congress empowered the Equal Employment Opportunity Commission to punish discrimination. The Detroit auto maker, employing thousands of minorities in factories, started recruiting African-American college graduates for professional roles.

Thomas Kimble, an accounting student from historically black Bishop College in Texas, was an initial recruit. Bishop has long since closed and Mr. Kimble has retired. But Mr. Kimble's own account of his career charts the auto giant's imperfect pursuit of culture change.

In the early 1970s, GM was what Apple Inc. is today; the car business was Silicon Valley. Jobs in the industry's design houses, engineering centers, finance departments and sales offices were coveted. Ivy leaguers eagerly relocated to the Motor City. Several blue chips, including DuPont, recruited Mr. Kimble, but back then few turned down The General.

"It was the largest industrial corporation in the world, nobody was even close," Mr. Kimble told me Tuesday. "If I can make it at General Motors, I can make it anywhere in the world."

He first joined the truck-and-bus unit in Pontiac, Mich., the first black professional to land on its finance staff. He figured the discrimination he encountered growing up in segregated Dallas -- where his parents taught him to avoid looking white people in the eye for fear of confrontation -- would be more subtle.

Freshly hired white counterparts scored apartments with ease, but no one would rent to GM's new black professionals. Ed Welburn, John Littleton and Tyrone Brown -- black men also hired in 1972 -- told me stories of routinely being turned down in various Detroit suburbs.

"Some of them were honest enough to say we don't rent apartments to Afro-Americans," Mr. Kimble recalled. All four men lived in a small dorm on the campus of Oakland University in Rochester during their first weeks on the job.

Racism was entrenched at GM, too. Companies can say all of the right words, recruit at historically black colleges and tie managers' pay to diversity, but bias has deep roots.

At lunchtime, Mr. Kimble's white colleagues would venture out together but never extended an invite. "It was very hurtful," he said. "It made you a very lonely person."

"I called my mom and told her I could not do it," he said of his early days at the company. "She said, 'You're going to do it.' "

When he started, Mr. Kimble was paired with a white accountant named Bob who constantly criticized him and piled paperwork on his desk. He figured Bob was his boss, but it turned out that Bob was just a colleague. It wasn't until two months into the job that a secretary told Mr. Kimble who his real supervisor was. The boss said Mr. Kimble should be the one telling Bob what to do, not the other way around.

Mr. Kimble, still in his first year, was assigned to join a team of GM "high pots," high-potential employees, to analyze machinery that kept breaking down. He wasn't a high-pot, but he was black, and his involvement would help managers meet diversity targets tied to bonuses.

"If they got 10 guys, then they've got to have a brother to fill their quota," Mr. Kimble said. "It wasn't a culture of the heart, it was a culture to protect the paycheck."

He seized the moment, though. After he and the others had independently conducted their analyses, they were called to present findings to the comptroller. One by one, white colleagues presented estimates to fix the machine in the $1 million range.

Mr. Kimble listened quietly, praying the comptroller wouldn't call on him. When his time came, he rose and said the solution would cost $50,000. Everyone laughed.

"I could just see it on their faces: 'We told you not to put this kid in there, this is so far over his head. This is what we talk about when we talk about affirmative action. These people aren't ready.'"

When asked to justify his conclusion, Mr. Kimble presented the business case that would change his career.

"Most of the time I don't go to lunch with these guys; I have to go out to the plant," he told the comptroller. "I asked them to show me where this piece of equipment was because I had to do a special analysis to replace the equipment. They told me, 'It's not the equipment, it's the tool that's attached to it that breaks down all the time.'"

The tool cost $25,000. He recommended buying two. The comptroller asked the high-pots why they never bothered to ask factory workers for advice. Mr. Kimble remembers him saying, 'This kid here just saved General Motors a million dollars, and the only thing that was wrong was a damn tool. Everybody get the f -- out of here."

He became known for his "million-dollar save," and forged a relationship with the comptroller. Promotions soon followed. Over 30 years, Mr. Kimble spearheaded diversity initiatives while running major operations. In the late 1990s, a black mentor named Bill Brooks promoted him to director of global philanthropy. He retired in 2002 as vice chairman of the GM Foundation.

Mr. Welburn, who joined the same summer as Mr. Kimble, rose to the top of a vaunted GM design department once run by legends like Harley Earl before he retired in 2016. He spent decades being respected but was never mentored until later in his career. He told me if his predecessor, Wayne Cherry, had not taken him under his wing he probably wouldn't have gotten the top job.

For every Kimble and Welburn, however, there is a Brown and Littleton. Messrs. Brown and Littleton both left GM with bad tastes in their mouths.

Mr. Littleton, a Michigan State grad with a master's degree and many allies -- including his wife -- in the company, waited more than a decade to be promoted to management. He was routinely passed over for jobs that he felt qualified for.

He said he didn't experience equity in the workplace until he joined Electronic Data Systems in 1995, when GM spun off the unit that it had acquired from billionaire Ross Perot a decade earlier. Mr. Littleton became a director at EDS, mentored younger black and white employees and said he thrived in the company even after Hewlett-Packard Co. bought EDS in 2008.

Mr. Brown, a commercial artist educated at Florida A&M, left GM in the mid-1980s after experiencing what he describes as a nervous breakdown. He struggled to find advocates while at GM and was demoralized by what he saw as a double standard.

While never explicitly told he was held back because of the color of his skin, he said he experienced barriers to pay increases and advancement no matter how well he performed.

"You had all these posters hanging everywhere saying 'we are fair and inclusive,' but the white guys didn't see it that way."

Mr. Brown continues to work, independently consulting on projects in the Detroit area.

"I had to forgive and forget General Motors," he said. "It was not an easy career path."

Write to John D. Stoll at john.stoll@wsj.com