California became the first state in the country to require that women be included on companies’ boards of directors, as Gov. Jerry Brown literally sent a message to Washington on Sunday in signing legislation that corporate associations opposed as unconstitutional.

Brown signed SB826 into law after it passed the Assembly and the Senate last month. The bill mandates that all publicly traded California companies have at least one woman on their boards by the end of 2019.

The requirement ramps up in 2021: Five-member boards will be expected to have two female members, and boards with six or more members will be expected to have three.

In a signing statement, Brown acknowledged that “serious legal objections have been raised” about the bill. “I don’t minimize the potential flaw that indeed may prove fatal to its ultimate implementation.”

However, Brown added, “recent events in Washington, D.C. — and beyond — make it crystal clear that many are not getting the message. ... Given all the special privileges that corporations have enjoyed for so long, it’s high time corporate boards include the people who constitute more than half of all ‘persons’ in America.”

Now Playing:

Brown CC’ed his signing message to the Senate Judiciary Committee, which last week heard Palo Alto University psychology Professor Christine Blasey Ford accuse Supreme Court nominee Brett Kavanaugh of trying to rape her when they were teenagers, along with Kavanaugh’s angry denial.

Democratic Sen. Connie Leyva of Chino, a co-author of the bill, said, “We know that when boards are diverse and women’s voices are heard, it’s better for the whole workforce.” She said the bill sends “a big message to women that we value them, we respect them here in California.”

Co-author Sen. Hannah-Beth Jackson, D-Santa Barbara, added, “Yet another glass ceiling is shattered, and women will finally have a seat at the table in corporate boardrooms.”

The bill cited studies by MSCI, Credit Suisse and UC Berkeley that found that companies with women on their boards have greater earnings per share, return on equity and transparency.

As of late August, women held just 546, or 15.8 percent, of the 3,445 seats on boards of directors at the 467 publicly traded California companies in the Russell 300 index, according to Bloomberg.

SB826 applies to companies whose main offices are in California, and also foreign companies whose U.S. headquarters in the state.

It may leave some companies scrambling to find female board members. Opponents include the California Chamber of Commerce, the California Restaurant Association and the California Ambulance Association.

“CAA is opposed to SB 826 because it is likely unconstitutional and favors one element of a diverse workforce over all others,” Chris Micheli, spokesman for the ambulance association, said in a statement. “Gender is an important aspect of board diversity, but the state should not elevate this element over all other aspects of diversity.”

Not all companies opposed the requirement. Zendesk, a customer service software company in San Francisco, has met even the 2021 requirements of SB826, with women making up half of its board of directors. Zendesk spokeswoman Tiffany Apczynski characterized the gender makeup as an effort to reflect its desired global customer base.

“When you pass bills like this, what you’re asking a company to do is rethink their entire recruiting and retention processes,” Apczynski said. “The problem historically, if you look at any questions of affirmative action where you put quota in the requirement, is suddenly there’s this perception that you’re going to get (a) less than 100 percent candidate, like the quality is going to diminish.”

To find female board members, corporations could turn to hubs like the Boardlist, a San Francisco company founded by StubHub President Sukhinder Singh Cassidy, that connects qualified women candidates with board opportunities.

Rebecca Aydin is a San Francisco Chronicle staff writer. Email: business@sfchronicle.com