"Panel members will bring mature experience, most notably in combat and deployed operations, and seasoned judgment to this task," he said. "The panel will assemble and thoroughly analyze all pertinent data, quantifiable and nonquantifiable."
He said he will provide advice to Trump on how to implement the policy after the panel arrives at recommendations. "In the interim, current policy with respect to currently serving members will remain in place," said Mattis.
The move allows transgender service members to serve openly in the military until the panel concludes its study.
On Friday, Trump signed a memo that provided policy guidance to the Pentagon about on the ban, keeping in place a restriction barring openly transgender people from joining the military and ending resourcing for gender reassignment surgeries, except for people whose procedures are already underway, to protect their health.
The memo gives Mattis until Feb. 21, 2018, to come up with an implementation plan for reinstating the ban on transgender people in the military. The memo reads that he will "determine how to address transgender individuals currently serving in the United States military. Until the secretary has made that determination, no action may be taken against such individuals under the policy."
The memo directs him to take steps consistent with "military effectiveness and lethality, budgetary constraints and applicable law" to implement the plan. But it seems to provide Mattis with flexibility in interpreting those terms.
On Friday, a senior White House official declined to specify for reporters what factors Mattis could consider to determine the future of transgender service members who have been serving openly.
The Pentagon does not track how many transgender service members there are in the military, but about 250 service members are undergoing gender transition or have changed their gender in personnel records.
A 2016 Rand study estimated there might be 1,320 to 6,630 transgender service members on active duty.
In his statement Mattis said, "Our focus must always be on what is best for the military's combat effectiveness leading to victory on the battlefield."
"I expect to issue interim guidance to the force concerning the president's direction, including any necessary interim adjustments to procedures, to ensure the continued combat readiness of the force until our final policy on this subject is issued," he added.