"We have a very strong transatlantic bond," Mattis told NATO's secretary general.
"It's getting stronger, relationships don't stay the same, they are always changing. In this case the bond is getting stronger."
Secretary of State Rex Tillerson is missing NATO's meeting of foreign ministers in April, yet will travel to Russia the same month, fueling fears about Washington's commitment to the alliance.
Stoltenberg twice declined to comment when a reporter shouted questions about the message Tillerson's decision will send to NATO.
NATO countries were deeply unsettled by President Donald Trump's fiery campaign rhetoric in which he branded the 28-member alliance as "obsolete."
Trump has moderated his stance since, though has continued to chide some members for not paying a promised 2 percent of GDP into their defense budgets.
Stoltenberg is in Washington to attend a meeting of allies fighting the Islamic State group in Iraq and Syria.
While NATO is not itself part of that coalition, many of its members are and some want NATO to play a broader role.
After speaking with Mattis, Stoltenberg said in a statement that the two of them agreed that NATO has "untapped potential in fighting terrorism by training local forces and building their capacity, as we have done for years in Afghanistan and the Balkans."
Mattis, a retired Marine general who has worked closely with NATO, has been on something of a damage-control mission since Trump took office and traveled to Brussels last month to reassure allies of ongoing US commitment to the mutual defense organization.
"I was very much impressed by the increased unity I found during my visit with you and the resolve of the alliance," Mattis told Stoltenberg.
"We had all hoped for a better future back 20 years ago and some things haven't turned out the way we wish, so we have to address that reality."
Stoltenberg, who was formerly the Norwegian prime minister, also told Mattis that security in Europe benefits North America too.
"In times of turmoil, in times of uncertainty, the need for strong international institutions like NATO is even greater," Stoltenberg said.
"Stability in Europe is good for all of us. Two world wars and the Cold War have taught us all that peace and stability is important for Europe of course but also North America."
Eyewitness? Submit your stories now via social or:
Email: news@pulselive.co.ke
