Mattis, during a brief visit to Seoul, defended US President Donald Trump's decision this month to halt "war games" with South Korea, saying it would increase the opportunity for diplomats to negotiate.
But Mattis added that US and South Korean forces would remain "united, vigilant and ready."
Critics of Trump's decision on the joint military exercises say it is a major concession to North Korea that, if prolonged during what are expected to be lengthy negotiations, risk eroding the readiness of US and South Korean forces.
North Korea has long sought a suspension of the drills.
The decision is part of efforts to defuse tensions and build trust, Mattis and his South Korean counterpart Song Young-moo said in a joint statement after their talks.
"The two ministers agreed to continue exploring confidence and peace-building measures as long as North Korea continues dialogue in good faith," the statement said.
Trump has also drawn criticism from national security analysts for an agreement that emerged from his June 12 summit with North Korean leader Kim Jong-un that had few details on how Pyongyang would give up its nuclear weapons and ballistic missiles.
"US commitment to the Republic of Korea remains ironclad and the US will continue to use the full range of diplomatic and military capabilities to uphold this commitment,"Mattis said before meeting Song.
"And this includes maintaining the current US force levels on the Korean Peninsula," he said.
Still, Mattis has yet to explain how the US will maintain readiness of US-South Korean forces, who have long prided themselves on the ability of being able to "fight tonight" if needed.
Washington and Seoul have already suspended planning for the upcoming Freedom Guardian exercise, which last year involved 17,500 American troops and more than 50,000 South Korean troops.