Mattis -- whose embrace of NATO differs from comments made by President Donald Trump -- made his first official calls to Secretary General of NATO Jens Stoltenberg and to his counterparts from two key NATO allies, UK Defense Secretary Michael Fallon and Canadian Minister of Defense Harjit Sajjan.
According to the Pentagon's official readout, Mattis in his conversation with Fallon "emphasized the United States' unshakeable commitment to NATO."
White House press secretary Sean Spicer was asked about Mattis' comments at the press briefing Tuesday.
"The President is very clear that as it's structured now, in terms of the output of NATO, he doesn't feel as though it's doing what its mission was set up to do or that it's being particularly effective," Spicer said.
"I think that Secretary Mattis and others have some ideas about how to reform and make organizations like NATO more effective and benefit the amount of money that the United States is putting into whether it's NATO or the UN or other organizations like that," he added. "But I think (Trump) is not pleased in terms of what he sees in terms of the US contribution and the output it's getting."
The official statement issued after the call with Stoltenberg acknowledged the symbolism of the communication, saying that Mattis "wanted to place the call on his first full day in office to reinforce the importance he places on the alliance."
Stoltenberg referred to Mattis as "a strong friend of NATO at the Pentagon," noting that the retired general had served as NATO's Supreme Allied Commander for Transformation while still in uniform and because of that Mattis "knows the Alliance's value well."
With his Canadian counterpart, Mattis discussed Canada's role in leading one of the NATO battalions that will be deployed to Latvia along the NATO-Russia frontier, a move that has irked Moscow
. The US will command a similar battalion in Poland.
In an interview with the Times of London and Bild conducted days before his inauguration, Trump said NATO was obsolete "because it was, you know, designed many, many years ago," and "because it wasn't taking care of terror."
He also during the presidential campaign repeatedly slammed the 23 members of the alliance that are not meeting the alliance's recommended defense spending levels of 2% of GDP.
While Mattis calls could be seen as an attempt to assuage fears that the Trump administration might pull back from the decades-old military alliance, the statements following the conversations issued by Fallon and Stoltenberg also addressed some of the issues that Trump has spoken about, including that NATO is ill-suited to tackle terrorism and that members do not spend enough on defense.
"We talked of our joint leadership in NATO, including modernizing the Alliance and how we ensure that all members meet the NATO 2% spending commitment alongside America and Britain," Fallon said in a statement.
And the statement released by NATO said Mattis and Stoltenberg "looked forward to working together to strengthen the Alliance, including by increasing defense spending and doing even more to fight terrorism."
"Yesterday evening I had privilege of speaking to the new US Defense Secretary James Mattis. He expressed strong support for NATO. He is an old friend of NATO," Stoltenberg said Tuesday at the opening of a new cooperation center in Kuwait.
He said he was "absolutely certain that the new President -- President Trump -- and the new US administration will be fully committed to NATO and to the transatlantic partnership."
"European Allies have to spend more, invest more in defense. I look forward to working together with President Trump and Secretary Mattis in pushing this agenda and make progress when it comes to increased defense spending among European NATO Allies," he added.
Mattis will have the chance to continue reassuring NATO partners next month, as the alliance will host a meeting of defense ministers, which is likely to be Mattis's first trip abroad as Defense Secretary.
"I look forward to welcoming him back to the NATO family, and to seeing him at our next meeting of Defense Ministers in February," Stoltenberg said.