"Our Department's leadership, civilian and military, remains in the best possible hands," Mattis said.
Mattis said he remains confident that members of the Department of Defence will not be distracted from their mission to support and defend the US Constitution.
READ MORE: Trump Speaks About His Disagreements With Mattis, Former Defence Chief
The defence secretary resigned last month, saying he planned to step down at the end of February, but just days after the announcement, Trump said Mattis would leave by January 1.
Deputy Secretary of Defence Patrick Shanahan will take over as acting defence chief until Trump names a replacement for Mattis.