Still, the attorney for the plaintiffs said the decision puts thousands of federal employees within a step of receiving a payout.
"We find her decision encouraging," said Heidi R. Burakiewicz, the Washington attorney who represents the workers. "The remaining issue on the case is whether the clients are due liquidated damages. We feel very optimistic about that."
The Justice Department is defending the government. Spokeswoman Nicole A. Navas said the department declines to comment on pending litigation.
When the government shut down from Oct. 1 to Oct. 16, workers in positions deemed essential — such as prison guards, firefighters and border patrol agents — stayed on the job.
Congress passed legislation to ensure that members of the armed forces and civilian defense workers would continue to receive their paychecks. But lawmakers didn't make the same arrangements for others.
As many as 1.3 million employees didn't receive pay when they should have for working between Oct. 1 and Oct. 5, Burakiewicz said.
After the shutdown ended, they were paid retroactively. But plaintiffs say the delay caused them to miss payments on mortgages, credit cards and other loans.
They seek minimum wage for every hour they didn't receive pay on time, as well as overtime.
"People had to make arrangements with their creditors; they had to make arrangements with their banks," said Jeffrey Roberts, an electronics technician for the Federal Bureau of Prisons in Forrest City, Ark.
Roberts, 48, a plaintiff in the case, said many of the employees affected were not highly paid and depended on every dollar they were owed.
He said the shutdown and the delays in payments also hurt morale.
"I'm actually just disappointed," he said.
Working-class employees were put in financial peril because of politics, he said, and he was upset that Congress declined to protect federal prison workers who deal with "America's worst" when lawmakers had the "forethought" to spare service members and veterans.
Burakiewicz said federal prisoners taunted some of her clients during the shutdown, questioning the government's loyalty to them. She called the "low point of their careers."
In court filings, the Justice Department said the federal government didn't violate the Fair Labor Standards Act — designed to protect low-wage earners — because pay wasn't withheld, just delayed.
But Campbell-Smith wrote in her ruling that courts have found that delaying pay is a violation.
The federal government said it could not have paid workers on time because of the Anti-Deficiency Act, a law that forbids the government from spending money if Congress doesn't appropriate it.
Burakiewicz said that's not a reasonable defense because the decision to withhold paychecks was made arbitrarily for political reasons and not out of fiscal emergency.
"This was done on purpose," she said. "The Department of Justice has been put in a position of defending something that's indefensible, but my clients want to send a message that this is something they don't want to happen again."
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Burakiewicz said her office has filed a motion to have the case certified conditionally as a "collective action" so all employees who were affected by the shutdown can be notified about the lawsuit and can decide whether to join the case.
The Department of Justice has until September to oppose the motion, she said.
The American Federation of Government Employees said it was "monitoring this litigation closely."
"If it appears that damages are available," the union said in a statement, "we will vigorously pursue an action for damages on behalf of the thousands of federal employees represented by AFGE who are entitled to a recovery."