Mattis refused to hint at any details of the decision, which came after months of speculation over whether Trump, frustrated with a stalemate after 16 years in Afghanistan, would allow the Pentagon to boost troop numbers on the ground in the country.
However Mattis appeared satisfied after what he described as an in-depth review of the policy by much of the president's cabinet and top security officials at Camp David on Friday.
"Everyone who had equity was heard," he said, including budget officials responsible for funding the effort.
Trump had several options on the table, that ranged from backing away from the country to stepping up US efforts to defeat the Taliban. In June he gave Mattis the power to increase troop numbers above the estimated 8,400 that have been in the country - close to 4,000 more, according to reports.
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"The president had to make strategic decisions," Mattis said.
"He delegated to me, when he came in, the tactical and operational decision. He did not delegate one ounce of the strategic decision."
"He really did come in with very different courses of action, and I think he now needs the weekend to collect his thoughts about how he's going to explain it to the American people."
In Turkey he will hold discussions with President Recep Tayyip Erdogan and top military officials focused on the Syria conflict and the fight against the Islamic State group. In Ukraine he will discuss US support for the country's military fighting pro-Moscow rebels and attend celebrations for the country's national day.