At the transfer ceremony, which was attended by Acting Defense Minister Patrick Shanahan, Mackenzie pledged to continue the work of his predecessor, General Joseph Votel, who was retired.
For his part, Votel said that at a time when Daesh was defeated and peace negotiations advanced with the Taliban, the US military presence in Syria and Afghanistan was still necessary.
"It is time to end these conflicts, and this will not be as fast as anyone wants, but it can be done if we keep our goals and interests in mind, document our partners and empower our forces," he stressed.
Mackenzie, 63, is familiar with the situation in Afghanistan where he served twice, as well as his participation in the Iraq war.
During his confirmation hearing at the beginning of December, he expressed concern about an early military withdrawal from Afghanistan, where local security forces took over from NATO in early 2015 to help fight the Taliban.