UK General David Richards has handed over control of the Nato-led force in Afghanistan to US General Dan McNeill at a ceremony in the capital, Kabul.
Gen McNeill has been based in Afghanistan before
Gen Richards headed the International Security Assistance Force (Isaf) for nine months, as troop levels increased from 9,000 to more than 33,000.
Isaf faces a fierce Taleban offensive in southern regions, and has asked for further reinforcements.
But so far only the US and Britain have pledged to send more troops.
It has been a challenging time for Gen Richards, the BBC's Alistair Leithead in Kabul says.
Gen Richards faced Taleban attacks in the south where US, British, Canadian and Dutch troops were deployed in areas previously outside the control of the government, our correspondent says.
He also made deals with local elders, which was followed in one area, Musa Qala, by the Taleban moving into an area recently vacated international troops.
Nevertheless, he marked the end of his term in charge of the Nato force by declaring that the conflict in Afghanistan is a "winnable war".
In a BBC interview, Gen Richards said the Taleban commander in the disputed town of Musa Qala in Helmand province, Mullah Abdul Ghafoor, had been killed in an air strike overnight.
"It's almost 100% certain that... the leader of the Taleban grouping in Musa Qala that intimidated the people and took over the centre, has just actually been taken out in an air strike along with some of his leading henchmen."
AFP news agency quoted Taleban spokesman Yousef Ahmadi as saying that some fighters had been killed in the strike, but denied that any "famous and big Taleban commander" was among them.
Speaking to the Associated Press, Gen Richard said the Taleban had failed to recover from a major battle last year.
"They know in a conventional sense their attempt, which I will give them credit for was a good attempt, failed when they gathered 1,000 fighters in one place."
The new commander will have more troops to deploy
Gen McNeill took command of the multinational headquarters in a ceremony on Sunday morning, and said his aim would be to improve the lives of ordinary Afghans.
"Our mission is to facilitate the reconstruction of Afghanistan."
Gen McNeill has been based in Afghanistan before, and will take on the Isaf mission of bringing security and development, and helping win over the people for the Afghan government, our correspondent says.
He says there has been some criticism from US quarters of the British policy of making peace deals with local elders.
The new commander is expected to have more troops available to him than Gen Richards could call upon.
But there is an uncertainty as to whether Gen McNeill might take a more direct approach to the counter-insurgency but that is yet to be seen, our correspondent says.