Wilma Liebman, a former chairwoman of the National Labor Relations Board under President Obama and now an occasional consultant to unions, said the decision could give fast-food workers and labor unions leverage to get the company to negotiate about steps that would make it easier to organize McDonald’s restaurants. Similarly, she said, the ruling could give the workers and unions more clout in pressing McDonald’s to have its franchisees raise wages.

And in an era when companies increasingly use subcontractors and temp agencies to free themselves of employment decisions and headaches, experts said the ruling could force the companies to be more accountable.

“Employers like McDonald’s seek to avoid recognizing the rights of their employees by claiming that they are not really their employer, despite exercising control over crucial aspects of the employment relationship,” said Julius Getman, a labor law professor at the University of Texas. “McDonald’s should no longer be able to hide behind its franchisees.”

The next phase will unfold before administrative law judges hearing the employees’ claims. If the judges rule against McDonald’s on the joint-employer finding and the labor violations claimed, the company is likely to appeal to the full five-member labor board in Washington. Given that McDonald’s is arguing that the legal counsel’s ruling goes against three decades of law, the case could ultimately wind up before the Supreme Court.

The cases filed with the N.L.R.B. grew out of the five one-day strikes demanding a $15 wage that fast-food workers conducted against McDonald’s and other fast-food restaurants beginning in November 2012. Over 100 workers complained to the board, saying that they had been fired, had their hours cut or were otherwise punished for the protests.

In a statement, Angelo Amador, vice president for labor and work force policy for the National Restaurant Association, called the ruling another example of the Obama administration’s agenda against small businesses. The decision, he said, “overturns 30 years of established law regarding the franchise model in the United States, erodes the proven franchisor-franchisee relationship, and jeopardizes the success of 90 percent of America’s restaurants who are independent operators or franchisees.”

In 1982, a federal appeals court, echoing the labor board at the time, said that a company was to be considered a joint employer in situations where two or more employers exerted “significant control” over the same employees. In the years since, the board has adopted a narrower standard, holding that a company could be deemed a joint employer only when it directly controlled, for instance, a franchisee’s or a temp agency’s employment practices.