The proportion of minorities being appointed last year matched the all-time high of 23 percent first achieved in 2017, mainly due to the more than doubling of African-American appointees, accounting for 11 percent of new board seats in 2018. Other ethnicities have a much tougher row to hoe: just over 4 percent for new Hispanic appointments, and 8 percent for new Asian and Asian-American board appointments.

"As these trends suggest, the story of increasing diversity on boards is far from simple and far from over," the authors write. "Based on 10 years' worth of data and our extensive experience working with boards around the world, we see real but still disappointing progress – and some straightforward ways to speed things up."

One way: don't just appoint current or former CEOs, but cast the net more widely. Last year, 60 percent of new board appointees were those who hold or have held the top post at other companies.

"While CEOs certainly bring valuable skills to the table, many outstanding general managers and divisional heads have just as much expertise in skills such as strategic orientation, operational ability, and P&L experience," the authors write.

Other worthwhile recruits: presidents of universities, retired public servants who have led large government agencies and retired career military officers, as well as executives and managers from private companies, according to the report.

"In addition, diligent searching can identify younger executives on the fast track to the top," the authors write. "As a group, they're generally more diverse than executives in the past, and they will furnish many of tomorrow's CEOs."

The report also found that newly appointed directors last year most frequently worked in the consumer sector (23 percent), surpassing financial services experience (21 percent), which had led in the previous four years. Consumer boards appointed the largest number of women overall in 2018, although those appointments still totaled only 38 percent of all new board seats in the consumer industry, down from 47 percent in 2017.

"For the first time, we see companies are seeking directors with consumer experience more than any other industry, and this will be an interesting trend line to watch," says Bonnie Gwin, vice chairman and co-managing partner of the CEO and board practice at Heidrick & Struggles. "Taken as a whole, these results are encouraging, but there's still much work to be done to reach gender parity by 2023 and to create more diverse boards," Gwin says.

Read more:

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• Women dominate among HR executives