The GAO, a watchdog and non-political arm of Congress, said the decision violated the Antideficiency Act, which prohibits the executive government from spending money that Congress hasn't approved.
The decision was one of several ways the administration sought to ease the pain or lessen the impacts of the shutdown. For example, it also tapped into special funding for national park improvements to bring in maintenance and other workers to keep some parks operating.
The shutdown was triggered in December 2018 by President Donald Trump's demand that lawmakers include funding for a border wall during budget negotiations. It was resolved 35 days later after Congress and the President agreed to temporarily reopen the government while negotiations continued.
As the shutdown stretched on, about 42 million Supplemental Nutrition Assistance Program, also known as SNAP or food stamps, recipients stood at risk of losing their benefits, which are paid by the US Department of Agriculture to states, which distribute the funding.
Several weeks into the shutdown, Agriculture Secretary Sonny Perdue announced that department officials had found a way to pay out February benefits: It would cite a federal law allowing certain payments within 30 days of a shutdown beginning and send states the money early. It also set aside about $3 billion it had on hand for a contingency fund.
At the time, USDA said this was a "legally sound decision."
But the GAO disagreed. Its review found the "USDA's actions were improper" and said the department "should report a violation of the Antideficiency Act."
"Based on our statutory construction analysis and USDA's historical pattern of obligations, we conclude that section 110(b) did not authorize USDA to obligate amounts for the early payment of February SNAP benefits," the GAO finding reads.
The USDA did not immediately respond to a request for comment.
It is unclear if any administration officials or federal employees could face consequences for the decision. The GAO's guidelines on the Antideficiency Act lay out these potential consequences for violations: "Employees may be subject to appropriate administrative discipline including, when circumstances warrant, suspension from duty without pay or removal from office. In addition, employees may also be subject to fines, imprisonment, or both."