The step, coming after a delay of several months, gives NATO a more direct role in the war against Russia's invasion while stopping well short of committing its own forces.
Diplomats, however, acknowledge that the handover to NATO may have a limited effect given that the U.S. under Trump could still deal a major setback to Ukraine by slashing its support, as it is the alliance's dominant power and provides the majority of arms to Kyiv.
Trump, who will take office in January, has said he wants to end the war in Ukraine swiftly but not revealed how how he aims to do so. He has long criticised the scale of U.S. financial and military aid to Ukraine.
The headquarters of NATO's new Ukraine mission, dubbed NATO Security Assistance and Training for Ukraine (NSATU), is located at Clay Barracks, a U.S. base in the German town of Wiesbaden.
A person familiar with the matter told Reuters it was now fully operational. No public reason has been given for the delays.
In the past, the U.S.-led Ramstein group, an ad hoc coalition of some 50 nations named after a U.S. air base in Germany where it first met, has coordinated Western military supplies to Kyiv.
Polish defence minister Wladyslaw Kosiniak-Kamysz said last week in an interview with Reuters that after the NATO summit in Washington, the alliance took responsibility for the hub in Jasionka, Poland - the main transit hub for foreign aid to Ukraine - to a greater extent.
"The alliance decided that it will protect it to a greater extent; it will be more involved," said Kosiniak-Kamysz. He added that when the alliance takes responsibility, it also designates countries to take care of its protection.
In November, Germany offered the redeployment of its Patriot air defence systems to protect the hub in Poland.
Meanwhile, the outgoing Biden administration in Washington is scrambling to ship as many weapons as possible to Kyiv amid fears that Trump may cut deliveries of military hardware to Ukraine.
NSATU is set to have a total strength of about 700 personnel, including troops stationed at NATO's military headquarters SHAPE in Belgium and at logistics hubs in Poland and Romania.
Russia has condemned increases in Western military aid to Ukraine as risking a wider war.
